Rolled electroactive polymers

ABSTRACT

The invention describes rolled electroactive polymer devices. The invention also describes employment of these devices in a wide array of applications and methods for their fabrication. A rolled electroactive polymer device converts between electrical and mechanical energy; and includes a rolled electroactive polymer and at least two electrodes to provide the mechanical/electrical energy conversion. Prestrain is typically applied to the polymer. In one embodiment, a rolled electroactive polymer device employs a mechanism, such as a spring, that provides a force to prestrain the polymer. Since prestrain improves mechanical/electrical energy conversion for many electroactive polymers, the mechanism thus improves performance of the rolled electroactive polymer device.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/293,003 filed onMay 22, 2001, which is incorporated by reference for all purposes.

[0002] This application was made in part with government support undercontract number N00014-00-C-0497 awarded by the Office of NavalResearch. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to electroactive polymerdevices that convert between electrical energy and mechanical energy.More particularly, the present invention relates to rolled electroactive polymer devices and methods of fabricating these devices. In manyapplications, it is desirable to convert between electrical energy andmechanical energy. Exemplary applications requiring conversion fromelectrical to mechanical energy include robotics, pumps, speakers,sensors, microfluidics, shoes, general automation, disk drives, andprosthetic devices. These applications include one or more transducersthat convert electrical energy into mechanical work—on a macroscopic ormicroscopic level. Exemplary applications requiring conversion frommechanical to electrical energy include sensors and generators.

[0004] New high-performance polymers capable of converting electricalenergy to mechanical energy, and vice versa, are now available for awide range of energy conversion applications. One class of thesepolymers, electroactive elastomers, is gaining wider attention.Electroactive elastomers may exhibit high energy density, stress, andelectromechanical coupling efficiency. The performance of these polymersis notably increased when the polymers are prestrained in area. Forexample, a 10-fold to 25-fold increase in area significantly improvesperformance of many electroactive elastomers.

[0005] Conventionally, bulky and static frames are used to apply andmaintain prestrain for a single layer of electroactive polymer. Theframes also allow coupling between the polymer and the externalenvironment. These frames occupy significantly more space and weigh muchmore than a single polymer layer, and may compromise the energy densityand compact advantages that these new polymers provide.

[0006] Thus, improved techniques for implementing these high-performancepolymers would be desirable.

SUMMARY OF THE INVENTION

[0007] The present invention overcomes limitations in the prior art andprovides new rolled electroactive polymer devices. The present inventionalso includes employment of these devices in a wide array ofapplications and methods for their fabrication. A rolled electroactivepolymer device converts between electrical and mechanical energy; andincludes a rolled electroactive polymer and at least two electrodes toprovide the mechanical/electrical energy conversion. Prestrain may beapplied to the polymer. In one embodiment, a rolled electroactivepolymer device employs a mechanism, such as a spring, that provides aforce to strain the polymer. In one embodiment, the mechanism adds toany prestrain previously established in the polymer. In other cases, noprestrain is previously applied in the polymer and the mechanismestablishes prestain in the polymer. Since prestrain improvesmechanical/electrical energy conversion for many electroactive polymers,the mechanism thus improves performance of the rolled electroactivepolymer device. In addition, the mechanism may provide other benefitssuch as a varying force response with deflection, which may be tuned tothe needs of an application.

[0008] The rolled electroactive polymer transducer may be employed forone or more functions. When a suitable voltage is applied to electrodesin electrical communication with a rolled electroactive polymer, thepolymer deflects (actuation). This deflection may be used to domechanical work. Whether or not the polymer deflects, electrical statesimposed on the polymer may be used to vary the stiffness or dampingprovided by the polymer, which has various mechanical uses. When apreviously charged electroactive polymer deflects, the electric field inthe material is changed. The change in electric field may be used toproduce electrical energy—for generation or sensing purposes. Thus, somefunctions of use for an electroactive polymer include actuation,variable stiffness or damping, generation or sensing. Rolledelectroactive polymer devices allow for compact electroactive polymerdevice designs. The rolled devices provide a potentially highelectroactive polymer-to-structure weight ratio, and can be configuredto actuate in many ways including linear axial extension/contraction,bending, and multi-degree of freedom actuators that combine bothextension and bending. Rolled electroactive polymers of the presentinvention also provide a simple alternative for obtaining multilayerelectroactive polymer devices.

[0009] In one aspect, the present invention relates to a device forconverting between electrical and mechanical energy. The devicecomprises a transducer comprising at least two electrodes and a rolledelectroactive polymer in electrical communication with the at least twoelectrodes. The device also comprises a mechanism having a first elementoperably coupled to a first portion of the polymer and a second elementoperably coupled to a second portion of the polymer. The mechanismprovides a force that strains at least a portion of the polymer.

[0010] In another aspect, the present invention relates to a method forfabricating an electroactive polymer device. The method comprisesdisposing at least two electrodes on an electroactive polymer. Themethod also comprises rolling the electroactive polymer about a springto produce a rolled electroactive polymer. The method further comprisessecuring the rolled electroactive polymer to maintain its rolledconfiguration.

[0011] In yet another aspect, the present invention relates to a devicefor converting between electrical and mechanical energy. The devicecomprises a transducer comprising at least two electrodes and a rolledelectroactive polymer in electrical communication with the at least twoelectrodes. The device also comprises a spring having a first springportion operably coupled to a first portion of the polymer and a secondspring portion operably coupled to a second portion of the polymer.

[0012] These and other features and advantages of the present inventionwill be described in the following description of the invention andassociated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIGS. 1A and 1B illustrate a top view of a transducer portionbefore and after application of a voltage, respectively, in accordancewith one embodiment of the present invention.

[0014] FIGS. 2A-2D illustrate a rolled electroactive polymer device inaccordance with one embodiment of the present invention.

[0015]FIG. 2E illustrates an end piece for the rolled electroactivepolymer device of FIG. 2A in accordance with one embodiment of thepresent invention.

[0016]FIG. 3A illustrates a monolithic transducer comprising a pluralityof active areas on a single polymer in accordance with one embodiment ofthe present invention.

[0017]FIG. 3B illustrates a monolithic transducer comprising a pluralityof active areas on a single polymer, before rolling, in accordance withone embodiment of the present invention.

[0018]FIG. 3C illustrates a rolled transducer that producestwo-dimensional output in accordance with one environment of the presentinvention.

[0019]FIG. 3D illustrates the rolled transducer of FIG. 3C withactuation for one set of radially aligned active areas.

[0020] FIGS. 3E-G illustrate exemplary vertical cross-sectional views ofa nested electroactive polymer device in accordance with one embodimentof the present invention.

[0021] FIGS. 3H-J illustrate exemplary vertical cross-sectional views ofa nested electroactive polymer device in accordance with anotherembodiment of the present invention.

[0022]FIG. 3K illustrates a rolled electroactive polymer device thatallows a designer to vary the deflection vs. force profile of thedevice.

[0023]FIG. 4 illustrates an electrical schematic of an open loopvariable stiffness/damping system in accordance with one embodiment ofthe present invention.

[0024]FIG. 5A is block diagram of one or more active areas connected topower conditioning electronics.

[0025]FIG. 5B is a circuit schematic of a device employing a rolledelectroactive polymer transducer for one embodiment of the presentinvention.

[0026] FIGS. 6A-6D describe the manufacture of a rolled electroactivepolymer device in accordance with one embodiment of the presentinvention.

[0027]FIG. 7 is a schematic of a sensor employing a rolled electroactivepolymer transducer according to one embodiment of the present invention.

[0028] FIGS. 8A-8C illustrate the fabrication and implementation of amultilayer electroactive polymer device using rolling techniques inaccordance with one embodiment of the present invention.

[0029]FIGS. 8D and 8E illustrate side perspective views of the pushrodapplication from FIG. 8C before and after actuation, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention is described in detail with reference to afew preferred embodiments as illustrated in the accompanying drawings.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

[0031] 1. Electroactive Polymers

[0032] Before describing structures, fabrication and applications ofrolled electroactive polymers of the present invention, the basicprinciples of electroactive polymer construction and operation willfirst be illuminated. The transformation between electrical andmechanical energy in devices of the present invention is based on energyconversion of one or more active areas of an electroactive polymer.Electroactive polymers are capable of converting between mechanicalenergy and electrical energy. In some cases, an electroactive polymermay change electrical properties (for example, capacitance andresistance) with changing mechanical strain.

[0033] To help illustrate the performance of an electroactive polymer inconverting between electrical energy and mechanical energy, FIG. 1Aillustrates a top perspective view of a transducer portion 10 inaccordance with one embodiment of the present invention. The transducerportion 10 comprises a portion of an electroactive polymer 12 forconverting between electrical energy and mechanical energy. In oneembodiment, an electroactive polymer refers to a polymer that acts as aninsulating dielectric between two electrodes and may deflect uponapplication of a voltage difference between the two electrodes (a‘dielectric elastomer’). Top and bottom electrodes 14 and 16 areattached to the electroactive polymer 12 on its top and bottom surfaces,respectively, to provide a voltage difference across polymer 12, or toreceive electrical energy from the polymer 12. Polymer 12 may deflectwith a change in electric field provided by the top and bottomelectrodes 14 and 16. Deflection of the transducer portion 10 inresponse to a change in electric field provided by the electrodes 14 and16 is referred to as ‘actuation’. Actuation typically involves theconversion of electrical energy to mechanical energy. As polymer 12changes in size, the deflection may be used to produce mechanical work.

[0034]FIG. 1B illustrates a top perspective view of the transducerportion 10 including deflection. In general, deflection refers to anydisplacement, expansion, contraction, torsion, linear or area strain, orany other deformation of a portion of the polymer 12. For actuation, achange in electric field corresponding to the voltage difference appliedto or by the electrodes 14 and 16 produces mechanical pressure withinpolymer 12. In this case, the unlike electrical charges produced byelectrodes 14 and 16 attract each other and provide a compressive forcebetween electrodes 14 and 16 and an expansion force on polymer 12 inplanar directions 18 and 20, causing polymer 12 to compress betweenelectrodes 14 and 16 and stretch in the planar directions 18 and 20.

[0035] Electrodes 14 and 16 are compliant and change shape with polymer12. The configuration of polymer 12 and electrodes 14 and 16 providesfor increasing polymer 12 response with deflection. More specifically,as the transducer portion 10 deflects, compression of polymer 12 bringsthe opposite charges of electrodes 14 and 16 closer and the stretchingof polymer 12 separates similar charges in each electrode. In oneembodiment, one of the electrodes 14 and 16 is ground. For actuation,the transducer portion 10 generally continues to deflect untilmechanical forces balance the electrostatic forces driving thedeflection. The mechanical forces include elastic restoring forces ofthe polymer 12 material, the compliance of electrodes 14 and 16, and anyexternal resistance provided by a device and/or load coupled to thetransducer portion 10, etc. The deflection of the transducer portion 10as a result of an applied voltage may also depend on a number of otherfactors such as the polymer 12 dielectric constant and the size ofpolymer 12.

[0036] Electroactive polymers in accordance with the present inventionare capable of deflection in any direction. After application of avoltage between the electrodes 14 and 16, the electroactive polymer 12increases in size in both planar directions 18 and 20. In some cases,the electroactive polymer 12 is incompressible, e.g. has a substantiallyconstant volume under stress. In this case, the polymer 12 decreases inthickness as a result of the expansion in the planar directions 18 and20. It should be noted that the present invention is not limited toincompressible polymers and deflection of the polymer 12 may not conformto such a simple relationship.

[0037] Application of a relatively large voltage difference betweenelectrodes 14 and 16 on the transducer portion 10 shown in FIG. 1A willcause transducer portion 10 to change to a thinner, larger area shape asshown in FIG. 1B. In this manner, the transducer portion 10 convertselectrical energy to mechanical energy. The transducer portion 10 mayalso be used to convert mechanical energy to electrical energy.

[0038] For actuation, the transducer portion 10 generally continues todeflect until mechanical forces balance the electrostatic forces drivingthe deflection. The mechanical forces include elastic restoring forcesof the polymer 12 material, the compliance of electrodes 14 and 16, andany external resistance provided by a device and/or load coupled to thetransducer portion 10, etc. The deflection of the transducer portion 10as a result of an applied voltage may also depend on a number of otherfactors such as the polymer 12 dielectric constant and the size ofpolymer 12.

[0039] In one embodiment, electroactive polymer 12 is pre-strained.Pre-strain of a polymer may be described, in one or more directions, asthe change in dimension in a direction after pre-straining relative tothe dimension in that direction before pre-straining. The pre-strain maycomprise elastic deformation of polymer 12 and be formed, for example,by stretching the polymer in tension and fixing one or more of the edgeswhile stretched. Alternatively, as will be described in greater detailbelow, a mechanism such as a spring may be coupled to different portionsof an electroactive polymer and provide a force that strains a portionof the polymer. For many polymers, pre-strain improves conversionbetween electrical and mechanical energy. The improved mechanicalresponse enables greater mechanical work for an electroactive polymer,e.g., larger deflections and actuation pressures. In one embodiment,prestrain improves the dielectric strength of the polymer. In anotherembodiment, the prestrain is elastic. After actuation, an elasticallypre-strained polymer could, in principle, be unfixed and return to itsoriginal state.

[0040] In one embodiment, pre-strain is applied uniformly over a portionof polymer 12 to produce an isotropic pre-strained polymer. By way ofexample, an acrylic elastomeric polymer may be stretched by 200 to 400percent in both planar directions. In another embodiment, pre-strain isapplied unequally in different directions for a portion of polymer 12 toproduce an anisotropic pre-strained polymer. In this case, polymer 12may deflect greater in one direction than another when actuated. Whilenot wishing to be bound by theory, it is believed that pre-straining apolymer in one direction may increase the stiffness of the polymer inthe pre-strain direction. Correspondingly, the polymer is relativelystiffer in the high pre-strain direction and more compliant in the lowpre-strain direction and, upon actuation, more deflection occurs in thelow pre-strain direction. In one embodiment, the deflection in direction18 of transducer portion 10 can be enhanced by exploiting largeprestrain in the perpendicular direction 20. For example, an acrylicelastomeric polymer used as the transducer portion 10 may be stretchedby 10 percent in direction 18 and by 500 percent in the perpendiculardirection 20. The quantity of pre-strain for a polymer may be based onthe polymer material and the desired performance of the polymer in anapplication. Prestrain suitable for use with the present invention isfurther described in commonly owned, copending U.S. patent applicationSer. No. 09/619,848, which is incorporated by reference for allpurposes.

[0041] Generally, after the polymer is pre-strained, it may be fixed toone or more objects or mechanisms. For a rigid object, the object ispreferably suitably stiff to maintain the level of pre-strain desired inthe polymer. A spring or other suitable mechanism that provides a forceto strain the polymer may add to any prestrain previously established inthe polymer before attachment to the spring or mechanisms, or may beresponsible for all the prestrain in the polymer. The polymer may befixed to the one or more objects or mechanisms according to anyconventional method known in the art such as a chemical adhesive, anadhesive layer or material, mechanical attachment, etc.

[0042] Transducers and pre-strained polymers of the present inventionare not limited to any particular rolled geometry or type of deflection.For example, the polymer and electrodes may be formed into any geometryor shape including tubes and multi-layer rolls, rolled polymers attachedbetween multiple rigid structures, rolled polymers attached across aframe of any geometry—including curved or complex geometries, across aframe having one or more joints, etc. Deflection of a transduceraccording to the present invention includes linear expansion andcompression in one or more directions, bending, axial deflection whenthe polymer is rolled, deflection out of a hole provided on an outercylindrical around the polymer, etc. Deflection of a transducer may beaffected by how the polymer is constrained by a frame or rigidstructures attached to the polymer.

[0043] Materials suitable for use as an electroactive polymer with thepresent invention may include any substantially insulating polymer orrubber (or combination thereof) that deforms in response to anelectrostatic force or whose deformation results in a change in electricfield. One suitable material is NuSil CF19-2186 as provided by NuSilTechnology of Carpenteria, Calif. Other exemplary materials suitable foruse as a pre-strained polymer include silicone elastomers, acrylicelastomers such as VHB 4910 acrylic elastomer as produced by 3MCorporation of St. Paul, Minn., polyurethanes, thermoplastic elastomers,copolymers comprising PVDF, pressure-sensitive adhesives,fluoroelastomers, polymers comprising silicone and acrylic moieties, andthe like. Polymers comprising silicone and acrylic moieties may includecopolymers comprising silicone and acrylic moieties, polymer blendscomprising a silicone elastomer and an acrylic elastomer, for example.Combinations of some of these materials may also be used as theelectroactive polymer in transducers of this invention.

[0044] Materials used as an electroactive polymer may be selected basedon one or more material properties such as a high electrical breakdownstrength, a low modulus of elasticity (for large or small deformations),a high dielectric constant, etc. In one embodiment, the polymer isselected such that is has an elastic modulus at most about 100 MPa. Inanother embodiment, the polymer is selected such that is has a maximumactuation pressure between about 0.05 MPa and about 10 MPa, andpreferably between about 0.3 MPa and about 3 MPa. In another embodiment,the polymer is selected such that is has a dielectric constant betweenabout 2 and about 20, and preferably between about 2.5 and about 12.

[0045] An electroactive polymer layer in transducers of the presentinvention may have a wide range of thicknesses. In one embodiment,polymer thickness may range between about 1 micrometer and 2millimeters. Polymer thickness may be reduced by stretching the film inone or both planar directions. In many cases, electroactive polymers ofthe present invention may be fabricated and implemented as thin films.Thicknesses suitable for these thin films may be below 50 micrometers.

[0046] As electroactive polymers of the present invention may deflect athigh strains, electrodes attached to the polymers should also deflectwithout compromising mechanical or electrical performance. Generally,electrodes suitable for use with the present invention may be of anyshape and material provided that they are able to supply a suitablevoltage to, or receive a suitable voltage from, an electroactivepolymer. The voltage may be either constant or varying over time. In oneembodiment, the electrodes adhere to a surface of the polymer.Electrodes adhering to the polymer are preferably compliant and conformto the changing shape of the polymer. Correspondingly, the presentinvention may include compliant electrodes that conform to the shape ofan electroactive polymer to which they are attached. The electrodes maybe only applied to a portion of an electroactive polymer and define anactive area according to their geometry. Several examples of electrodesthat only cover a portion of an electroactive polymer will be describedin further detail below.

[0047] Various types of electrodes suitable for use with the presentinvention are described in commonly owned, copending U.S. patentapplication Ser. No. 09/619,848, which was previously incorporated byreference above. Electrodes described therein and suitable for use withthe present invention include structured electrodes comprising metaltraces and charge distribution layers, textured electrodes comprisingvarying out of plane dimensions, conductive greases such as carbongreases or silver greases, colloidal suspensions, high aspect ratioconductive materials such as carbon fibrils and carbon nanotubes, andmixtures of ionically conductive materials.

[0048] Materials used for electrodes of the present invention may vary.Suitable materials used in an electrode may include graphite, carbonblack, colloidal suspensions, thin metals including silver and gold,silver filled and carbon filled gels and polymers, and ionically orelectronically conductive polymers. In a specific embodiment, anelectrode suitable for use with the present invention comprises 80percent carbon grease and 20 percent carbon black in a silicone rubberbinder such as Stockwell RTV60-CON as produced by Stockwell Rubber Co.Inc. of Philadelphia, Pa. The carbon grease is of the type such asNyoGel 756G as provided by Nye Lubricant Inc. of Fairhaven, Mass. Theconductive grease may also be mixed with an elastomer, such as siliconelastomer RTV 118 as produced by General Electric of Waterford, N.Y., toprovide a gel-like conductive grease.

[0049] It is understood that certain electrode materials may work wellwith particular polymers and may not work as well for others. By way ofexample, carbon fibrils work well with acrylic elastomer polymers whilenot as well with silicone polymers. For most transducers, desirableproperties for the compliant electrode may include one or more of thefollowing: low modulus of elasticity, low mechanical damping, lowsurface resistivity, uniform resistivity, chemical and environmentalstability, chemical compatibility with the electroactive polymer, goodadherence to the electroactive polymer, and the ability to form smoothsurfaces. In some cases, a transducer of the present invention mayimplement two different types of electrodes, e.g. a different electrodetype for each active area or different electrode types on opposing sidesof a polymer.

[0050]2. Rolled Electroactive Polymer Devices

[0051] FIGS. 2A-2D show a rolled electroactive polymer device 20 inaccordance with one embodiment of the present invention. FIG. 2Aillustrates a side view of device 20. FIG. 2B illustrates an axial viewof device 20 from the top end. FIG. 2C illustrates an axial view ofdevice 20 taken through cross section A-A. FIG. 2D illustratescomponents of device 20 before rolling. Device 20 comprises a rolledelectroactive polymer 22, spring 24, end pieces 27 and 28, and variousfabrication components used to hold device 20 together.

[0052] As illustrated in FIG. 2C, electroactive polymer 22 is rolled. Inone embodiment, a rolled electroactive polymer refers to anelectroactive polymer with, or without electrodes, wrapped round andround onto itself (e.g., like a poster) or wrapped around another object(e.g., spring 24). The polymer may be wound repeatedly and at the veryleast comprises an outer layer portion of the polymer overlapping atleast an inner layer portion of the polymer. In one embodiment, a rolledelectroactive polymer refers to a spirally wound electroactive polymerwrapped around an object or center. As the term is used herein, rolledis independent of how the polymer achieves its rolled configuration.

[0053] As illustrated by FIGS. 2C and 2D, electroactive polymer 22 isrolled around the outside of spring 24. Spring 24 provides a force thatstrains at least a portion of polymer 22. The top end 24 a of spring 24is attached to rigid endpiece 27. Likewise, the bottom end 24 b ofspring 24 is attached to rigid endpiece 28. The top edge 22 a of polymer22 (FIG. 2D) is wound about endpiece 27 and attached thereto using asuitable adhesive. The bottom edge 22 b of polymer 22 is wound aboutendpiece 28 and attached thereto using an adhesive. Thus, the top end 24a of spring 24 is operably coupled to the top edge 22 a of polymer 22 inthat deflection of top end 24 a corresponds to deflection of the topedge 22 a of polymer 22. Likewise, the bottom end 24 b of spring 24 isoperably coupled to the bottom edge 22 b of polymer 22 and deflectionbottom end 24 b corresponds to deflection of the bottom edge 22 b ofpolymer 22. Polymer 22 and spring 24 are capable of deflection betweentheir respective bottom top portions.

[0054] As mentioned above, many electroactive polymers perform betterwhen prestrained. For example, some polymers exhibit a higher breakdownelectric field strength, electrically actuated strain, and energydensity when prestrained. Spring 24 of device 20 provides forces thatresult in both circumferential and axial prestrain onto polymer 22.

[0055] Spring 24 is a compression spring that provides an outward forcein opposing axial directions (FIG. 2A) that axially stretches polymer 22and strains polymer 22 in an axial direction. Thus, spring 24 holdspolymer 22 in tension in axial direction 35. In one embodiment, polymer22 has an axial prestrain in direction 35 from about 50 to about 300percent. As will be described in further detail below for fabrication,device 20 may be fabricated by rolling a prestrained electroactivepolymer film around spring 24 while it the spring is compressed. Oncereleased, spring 24 holds the polymer 22 in tensile strain to achieveaxial prestrain.

[0056] Spring 24 also maintains circumferential prestrain on polymer 22.The prestrain may be established in polymer 22 longitudinally indirection 33 (FIG. 2D) before the polymer is rolled about spring 24.Techniques to establish prestrain in this direction during fabricationwill be described in greater detail below. Fixing or securing thepolymer after rolling, along with the substantially constant outerdimensions for spring 24, maintains the circumferential prestrain aboutspring 24. In one embodiment, polymer 22 has a circumferential prestrainfrom about 100 to about 500 percent. In many cases, spring 24 providesforces that result in anisotropic prestrain on polymer 22.

[0057] End pieces 27 and 28 are attached to opposite ends of rolledelectroactive polymer 22 and spring 24. FIG. 2E illustrates a side viewof end piece 27 in accordance with one embodiment of the presentinvention. Endpiece 27 is a circular structure that comprises an outerflange 27 a, an interface portion 27 b, and an inner hole 27 c.Interface portion 27 b preferably has the same outer diameter as spring24. The edges of interface portion 27 b may also be rounded to preventpolymer damage. Inner hole 27 c is circular and passes through thecenter of endpiece 27, from the top end to the bottom outer end thatincludes outer flange 27 a. In a specific embodiment, endpiece 27comprises aluminum, magnesium or another machine metal. Inner hole 27 cis defined by a hole machined or similarly fabricated within endpiece27. In a specific embodiment, endpiece 27 comprises ½ inch end caps witha ⅜ inch inner hole 27 c.

[0058] In one embodiment, polymer 22 does not extend all the way toouter flange 27 a and a gap 29 is left between the outer portion edge ofpolymer 22 and the inside surface of outer flange 27 a. As will bedescribed in further detail below, an adhesive or glue may be added tothe rolled electroactive polymer device to maintain its rolledconfiguration. Gap 29 provides a dedicated space on endpiece 27 for anadhesive or glue than the buildup to the outer diameter of the rolleddevice and fix to all polymer layers in the roll to endpiece 27. In aspecific embodiment, gap 29 is between about 0 mm and about 5 mm.

[0059] The portions of electroactive polymer 22 and spring 24 betweenend pieces 27 and 28 may be considered active to their functionalpurposes. Thus, end pieces 27 and 28 define an active region 32 ofdevice 20 (FIG. 2A). End pieces 27 and 28 provide a common structure forattachment with spring 24 and with polymer 22. In addition, each endpiece 27 and 28 permits external mechanical and detachable coupling todevice 20. For example, device 20 may be employed in a roboticapplication where endpiece 27 is attached to an upstream link in a robotand endpiece 28 is attached to a downstream link in the robot. Actuationof electroactive polymer 22 then moves the downstream link relative tothe upstream link as determined by the degree of freedom between the twolinks (e.g., rotation of link 2 about a pin joint on link 1).

[0060] In a specific embodiment, inner hole 27 c comprises an internalthread capable of threaded interface with a threaded member, such as ascrew or threaded bolt. The internal thread permits detachablemechanical attachment to one end of device 20. For example, a screw maybe threaded into the internal thread within end piece 27 for externalattachment to a robotic element. For detachable mechanical attachmentinternal to device 20, a nut or bolt to be threaded into each end piece27 and 28 and pass through the axial core of spring 24, thereby fixingthe two end pieces 27 and 28 to each other. This allows device 20 to beheld in any state of deflection, such as a fully compressed state usefulduring rolling. This may also be useful during storage of device 20 sothat polymer 22 is not strained in storage.

[0061] In one embodiment, a stiff member or linear guide 30 is disposedwithin the spring core of spring 24. Since the polymer 22 in spring 24is substantially compliant between end pieces 27 and 28, device 20allows for both axial deflection along direction 35 and bending ofpolymer 22 and spring 24 away from its linear axis (the axis passingthrough the center of spring 24). In some embodiments, only axialdeflection is desired. Linear guide 30 prevents bending of device 20between end pieces 27 and 28 about the linear axis. Preferably, linearguide 30 does not interfere with the axial deflection of device 20. Forexample, linear guide 30 preferably does not introduce frictionalresistance between itself and any portion of spring 24. With linearguide 30, or any other suitable constraint that prevents motion outsideof axial direction 35, device 20 may act as a linear actuator orgenerator with output strictly in direction 35. Linear guide 30 may becomprised of any suitably stiff material such as wood, plastic, metal,etc.

[0062] Polymer 22 is wound repeatedly about spring 22. For singleelectroactive polymer layer construction, a rolled electroactive polymerof the present invention may comprise between about 2 and about 200layers. In this case, a layer refers to the number of polymer films orsheets encountered in a radial cross-section of a rolled polymer. Insome cases, a rolled polymer comprises between about 5 and about 100layers. In a specific embodiment, a rolled electroactive polymercomprises between about 15 and about 50 layers.

[0063] In another embodiment, a rolled electroactive polymer employs amultilayer structure. The multilayer structure comprises multiplepolymer layers disposed on each other before rolling or winding. Forexample, a second electroactive polymer layer, without electrodespatterned thereon, may be disposed on an electroactive polymer havingelectrodes patterned on both sides. The electrode immediately betweenthe two polymers services both polymer surfaces in immediate contact.After rolling, the electrode on the bottom side of the electrodedpolymer then contacts the top side of the non-electroded polymer. Inthis manner, the second electroactive polymer with no electrodespatterned thereon uses the two electrodes on the first electrodedpolymer.

[0064] Other multilayer constructions are possible. For example, amultilayer construction may comprise any even number of polymer layersin which the odd number polymer layers are electroded and the evennumber polymer layers are not. The upper surface of the topnon-electroded polymer then relies on the electrode on the bottom of thestack after rolling. Multilayer constructions having 2, 4, 6, 8, etc.,are possible this technique. In some cases, the number of layers used ina multilayer construction may be limited by the dimensions of the rolland thickness of polymer layers. As the roll radius decreases, thenumber of permissible layers typically decrease is well. Regardless ofthe number of layers used, the rolled transducer is configured such thata given polarity electrode does not touch an electrode of oppositepolarity. In one embodiment, multiple layers are each individuallyelectroded and every other polymer layer is flipped before rolling suchthat electrodes in contact each other after rolling are of a similarvoltage or polarity.

[0065] The multilayer polymer stack may also comprise more than one typeof polymer For example, one or more layers of a second polymer may beused to modify the elasticity or stiffness of the rolled electroactivepolymer layers. This polymer may or may not be active in thecharging/discharging during the actuation. When a non-active polymerlayer is employed, the number of polymer layers may be odd. The secondpolymer may also be another type of electroactive polymer that variesthe performance of the rolled product.

[0066] In one embodiment, the outermost layer of a rolled electroactivepolymer does not comprise an electrode disposed thereon. This may bedone to provide a layer of mechanical protection, or to electricallyisolate electrodes on the next inner layer.

[0067] Device 20 provides a compact electroactive polymer devicestructure and improves overall electroactive polymer device performanceover conventional electroactive polymer devices. For example, themultilayer structure of device 20 modulates the overall spring constantof the device relative to each of the individual polymer layers. Inaddition, the increased stiffness of the device achieved via spring 24increases the stiffness of device 20 and allows for faster response inactuation, if desired.

[0068] In a specific embodiment, spring 24 is a compression spring suchas catalog number 11422 as provided by Century Spring of Los Angeles,Calif. This spring is characterized by a spring force of 0.91 lb/inchand dimensions of 4.38 inch free length, 1.17 inch solid length, 0.360inch outside diameter, 0.3 inch inside diameter. In this case, rolledelectroactive polymer device 20 has a height 36 from about 5 to about 7cm, a diameter 37 of about 0.8 to about 1.2 cm, and an active regionbetween end pieces of about 4 to about 5 cm. The polymer ischaracterized by a circumferential prestrain from about 300 to about 500percent and axial prestrain (including force contributions by spring 24)from about 150 to about 250 percent.

[0069] Device 20 has many functional uses. As will be described infurther detail below, electroactive polymers of the present inventionmay be used for actuation, generation, sensing, variable stiffness anddamping, or combinations thereof. Thus, device 20 may be used in roboticapplication for actuation and production of mechanical energy.Alternatively, rolled device 20 may contribute to stiffness and dampingcontrol of a robotic link. Thus, either end piece 27 or 28 may becoupled to a potentially moving mechanical link to receive mechanicalenergy from the link and damp the motion. In this case, polymer 22converts this mechanical energy to electrical energy according totechniques described below.

[0070] Although device 20 is illustrated with a single spring 24disposed internal to the rolled polymer, it is understood thatadditional structures such as another spring external to the polymer mayalso be used to provide strain and prestrain forces. These externalstructures may be attached to device 20 using end pieces 27 and 28 forexample.

[0071] The present invention also encompasses mechanisms, other than aspring, used in a rolled electraoctive polymer device to apply a forcethat strains a rolled polymer. As the term is used herein, a mechanismused to provide strain onto a rolled electroactive polymer generallyrefers to a system or an arrangement of elements that are capable ofproviding a force to different portions of a rolled electroactivepolymer. In many cases, the mechanism is flexible (e.g., a spring) orhas moving parts (e.g., a pneumatic cylinder). The mechanism may alsocomprises rigid parts (see the frame of FIG. 8B). Strain may also beachieved using mechanisms such as hydraulic actuators, pneumaticactuators, and magnetic systems (e.g., FIG. 3K), for example.Alternatively, compressible materials and foams may be disposed internalto the roll to provide the strain forces and allow for axial deflection.

[0072] Generally, the mechanism provides a force that onto the polymer.In one embodiment, the force changes the force vs. deflectioncharacteristics of the device, such as to provide a negative forceresponse, as described below. In another embodiment, the force strainsthe polymer. This latter case implies that the polymer deflects inresponse to the force, relative to its deflection state without theeffects of the mechanism. This strain may include prestrain as describedabove. In one embodiment, the mechanism maintains or adds to anyprestrain previously established in the polymer, such prestrain providedby a fixture during rolling as described below. In another embodiment,no prestrain is previously applied in the polymer and the mechanismestablishes prestain in the polymer.

[0073] In one embodiment, the mechanism is another elastomer that issimilar or different from the electroactive polymer. For example, thissecond elastomer may be disposed as a nearly-solid rubber core that isaxially compressed before rolling (to provide an axial tensile prestrainon the electroactive polymer). The elastomer core can have a thin holefor a rigid rod to facilitate the rolling process. If lubricated, therigid rod may be slid out from the roll after fabrication. One may alsomake a solid elastomer roll tightly wound with electroactive polymerusing a similar technique.

[0074] The mechanism and its constituent elements are typically operablycoupled to the polymer such that the strain is achieved. This mayinclude fixed or detachable coupling, permanent attachment, etc. In thecase of the spring above, operable coupling includes the use of anadhesive, such as glue, that attaches opposite ends of the spring toopposite ends of the polymer. An adhesive is also used to attach therolled polymer to the frame in FIG. 8B. The coupling may be direct orindirect, e.g., the magnet 252 of FIG. 3K is attached to the end piece242, which is attached to the rolled polymer. One of skill in the art isaware of numerous techniques to couple or attach two mechanicalstructures together, and these techniques are not expansively discussedherein for sake of brevity.

[0075] Rolled electroactive polymers of the present invention havenumerous advantages. Firstly, these designs provide a multilayer devicewithout having to individually frame each layer; and stack numerousframes (see FIG. 8B). In addition, the cylindrical package provided bythese devices is advantageous to some applications where long andcylindrical packaging is advantageous over flat packaging associatedwith planar electroactive polymer devices. In addition, using a largernumber of polymer layers in a roll improves reliability of the deviceand reduces sensitivity to imperfections and local cracks in anyindividual polymer layer.

[0076] 3. Alternate Rolled Electroactive Polymer Device Designs

[0077] 3.1 Multiple Active Areas

[0078] In some cases, electrodes cover a limited portion of anelectroactive polymer relative to the total area of the polymer. Thismay be done to prevent electrical breakdown around the edge of apolymer, to allow for polymer portions to facilitate a rolledconstruction (e.g., an outside polymer barrier layer), to providemultifunctionality, or to achieve customized deflections for one or moreportions of the polymer. As the term is used herein, an active area isdefined as a portion of a transducer comprising a portion of anelectroactive polymer and one or more electrodes that provide or receiveelectrical energy to or from the portion. The active area may be usedfor any of the functions described below. For actuation, the active areaincludes a portion of polymer having sufficient electrostatic force toenable deflection of the portion. For generation or sensing, the activearea includes a portion of polymer having sufficient deflection toenable a change in electrostatic energy. A polymer of the presentinvention may have multiple active areas.

[0079] In accordance with the present invention, the term “monolithic”is used herein to refer to electroactive polymers and transducerscomprising a plurality of active areas on a single polymer. FIG. 3Aillustrates a monolithic transducer 150 comprising a plurality of activeareas on a single polymer 151 in accordance with one embodiment of thepresent invention. The monolithic transducer 150 converts betweenelectrical energy and mechanical energy. The monolithic transducer 150comprises an electroactive polymer 151 having two active areas 152 a and152 b. Polymer 151 may be held in place using, for example, a rigidframe (not shown) attached at the edges of the polymer. Coupled toactive areas 152 a and 152 b are wires 153 that allow electricalcommunication between active areas 152 a and 152 b and allow electricalcommunication with communication electronics 155.

[0080] Active area 152 a has top and bottom electrodes 154 a and 154 bthat are attached to polymer 151 on its top and bottom surfaces 151 cand 151 d, respectively. Electrodes 154 a and 154 b provide or receiveelectrical energy across a portion 151 a of the polymer 151. Portion 151a may deflect with a change in electric field provided by the electrodes154 a and 154 b. For actuation, portion 151 a comprises the polymer 151between the electrodes 154 a and 154 b and any other portions of thepolymer 151 having sufficient electrostatic force to enable deflectionupon application of voltages using the electrodes 154 a and 154 b. Whenactive area 152 a is used as a generator to convert from electricalenergy to mechanical energy, deflection of the portion 151 a causes achange in electric field in the portion 151 a that is received as achange in voltage difference by the electrodes 154 a and 154 b.

[0081] Active area 152 b has top and bottom electrodes 156 a and 156 bthat are attached to the polymer 151 on its top and bottom surfaces 151c and 151 d, respectively. Electrodes 156 a and 156 b provide or receiveelectrical energy across a portion 151 b of the polymer 151. Portion 151b may deflect with a change in electric field provided by the electrodes156 a and 156 b. For actuation, portion 151 b comprises the polymer 151between the electrodes 156 a and 156 b and any other portions of thepolymer 151 having sufficient stress induced by the electrostatic forceto enable deflection upon application of voltages using the electrodes156 a and 156 b. When active area 152 b is used as a generator toconvert from electrical energy to mechanical energy, deflection of theportion 151 b causes a change in electric field in the portion 151 bthat is received as a change in voltage difference by the electrodes 156a and 156 b.

[0082] Active areas for an electroactive polymer may be easily patternedand configured using conventional electroactive polymer electrodefabrication techniques. Multiple active area polymers and transducersare further described in Ser. No. 09/779,203, which is incorporatedherein by reference for all purposes. Given the ability to pattern andindependently control multiple active areas allows rolled transducers ofthe present invention to be employed in many new applications; as wellas employed in existing applications in new ways.

[0083]FIG. 3B illustrates a monolithic transducer 170 comprising aplurality of active areas on a single polymer 172, before rolling, inaccordance with one embodiment of the present invention. Transducer 170comprises individual electrodes 174 on the facing polymer side 177. Theopposite side of polymer 172 (not shown) may include individualelectrodes that correspond in location to electrodes 174, or may includea common electrode that spans in area and services multiple or allelectrodes 174 and simplifies electrical communication. Active areas 176then comprise portions of polymer 172 between each individual electrode174 and the electrode on the opposite side of polymer 172, as determinedby the mode of operation of the active area. For actuation for example,active area 176 a for electrode 174 a includes a portion of polymer 172having sufficient electrostatic force to enable deflection of theportion, as described above.

[0084] Active areas 176 on transducer 170 may be configured for one ormore functions. In one embodiment, all active areas 176 are allconfigured for actuation. In another embodiment suitable for use withrobotic applications, one or two active areas 176 are configured forsensing while the remaining active areas 176 are configured foractuation. In this manner, a rolled electroactive polymer device usingtransducer 170 is capable of both actuation and sensing. Any activeareas designated for sensing may each include dedicated wiring tosensing electronics, as described below.

[0085] At shown, electrodes 174 a-d each include a wire 175 a-d attachedthereto that provides dedicated external electrical communication andpermits individual control for each active area 176 a-d. Electrodes 174e-i are all electrical communication with common electrode 177 and wire179 that provides common electrical communication with active areas 176e-i. Common electrode 177 simplifies electrical communication withmultiple active areas of a rolled electroactive polymer that areemployed to operate in a similar manner. In one embodiment, commonelectrode 177 comprises aluminum foil disposed on polymer 172 beforerolling. In one embodiment, common electrode 177 is a patternedelectrode of similar material to that used for electrodes 174 a-i, e.g.,carbon grease.

[0086] For example, a set of active areas may be employed for one ormore of actuation, generation, sensing, changing the stiffness and/ordamping, or a combination thereof. Suitable electrical control alsoallows a single active area to be used for more than one function. Forexample, active area 174 a may be used for actuation and variablestiffness control of a robotic limb in a robotics application. The sameactive area may also be used for generation to produce electrical energybased on motion of the robotic limb. Suitable electronics for each ofthese functions are described in further detail below. Active area 174 bmay also be flexibly used for actuation, generation, sensing, changingstiffness, or a combination thereof. Energy generated by one active areamay be provided to another active area, if desired by an application.Thus, rolled polymers and transducers of the present invention mayinclude active areas used as an actuator to convert from electrical tomechanical energy, a generator to convert from mechanical to electricalenergy, a sensor that detects a parameter, or a variable stiffnessand/or damping device that is used to control stiffness and/or damping,or combinations thereof.

[0087] In one embodiment, multiple active areas employed for actuationare wired in groups to provide graduated electrical control of forceand/or deflection output from a rolled electroactive polymer device. Forexample, a rolled electroactive polymer transducer many have 50 activeareas in which 20 active areas are coupled to one common electrode, 10active areas to a second common electrode, another 10 active areas to athird common electrode, 5 active areas to a fourth common electrode inthe remaining five individually wired. Suitable computer management andon-off control for each common electrode then allows graduated force anddeflection control for the rolled transducer using only binary on/offswitching. The biological analogy of this system is motor units found inmany mammalian muscular control systems. Obviously, any number of activeareas and common electrodes may be implemented in this manner to providea suitable mechanical output or graduated control system.

[0088] 3.2 Multiple Degree of Freedom Rolled Devices

[0089] In another embodiment, multiple active areas on an electroactivepolymer are disposed such subsets of the active areas radially alignafter rolling. For example, the multiple the active areas may bedisposed such that, after rolling, active areas are disposed every 90degrees in the roll. These radially aligned electrodes may then beactuated in unity to allow multiple degree of freedom motion for arolled electroactive polymer device.

[0090]FIG. 3C illustrates a rolled transducer 180 capable oftwo-dimensional output in accordance with one environment of the presentinvention. Transducer 180 comprises an electroactive polymer 182 rolledto provide ten layers. Each layer comprises four radially aligned activeareas. The center of each active area is disposed at a 90 degreeincrement relative to its neighbor. FIG. 3C shows the outermost layer ofpolymer 182 and radially aligned active areas 184, 186, and 188, whichare disposed such that their centers mark 90 degree increments relativeto each other. A fourth radially aligned active area (not shown) on thebackside of polymer 182 has a center approximately situated 180 degreesfrom radially aligned active area 186.

[0091] Radially aligned active area 184 may include common electricalcommunication with active areas on inner polymer layers having the sameradial alignment. Likewise, the other three radially aligned outeractive areas 182, 186, and the back active area not shown, may includecommon electrical communication with their inner layer counterparts. Inone embodiment, transducer 180 comprises four leads that provide commonactuation for each of the four radially aligned active area sets.

[0092]FIG. 3D illustrates transducer 180 with radially aligned activearea 188, and its corresponding radially aligned inner layer activeareas, actuated. Actuation of active area 188, and corresponding innerlayer active areas, results in axial expansion of transducer 188 on theopposite side of polymer 182. The result is lateral bending oftransducer 180, approximately 180 degrees from the center point ofactive area 188. The effect may also be measured by the deflection of atop portion 189 of transducer 180, which traces a radial arc from theresting position shown in FIG. 3C to his position at shown in FIG. 3D.Varying the amount of electrical energy provided to active area 188, andcorresponding inner layer active areas, controls the deflection of thetop portion 189 along this arc. Thus, top portion 189 of transducer 180may have a deflection as shown in FIG. 3D, or greater, or a deflectionminimally away from the position shown in FIG. 3C. Similar bending in ananother direction may be achieved by actuating any one of the otherradially aligned active area sets.

[0093] Combining actuation of the radially aligned active area setsproduces a two-dimensional space for deflection of top portion 189. Forexample, radially aligned active area sets 186 and 184 may be actuatedsimultaneously to produce deflection for the top portion in a 45 degreeangle corresponding to the coordinate system shown in FIG. 3C.Decreasing the amount of electrical energy provided to radially alignedactive area set 186 and increasing the amount of electrical energyprovided to radially aligned active area set 184 moves top portion 189closer to the zero degree mark. Suitable electrical control then allowstop portion 189 to trace a path for any angle from 0 to 360 degrees, orfollow variable paths in this two dimensional space.

[0094] Transducer 180 is also capable of three-dimensional deflection.Simultaneous actuation of active areas on all four sides of transducer180 will move top portion 189 upward. In other words, transducer 180 isalso a linear actuator capable of axial deflection based on simultaneousactuation of active areas on all sides of transducer 180. Coupling thislinear actuation with the differential actuation of radially alignedactive areas and their resulting two-dimensional deflection as justdescribed above, results in a three dimensional deflection space for thetop portion of transducer 180. Thus, suitable electrical control allowstop portion 189 to move both up and down as well as tracetwo-dimensional paths along this linear axis.

[0095] Although transducer 180 is shown for simplicity with fourradially aligned active area sets disposed at 90 degree increments, itis understood that transducers of the present invention capable of two-and three-dimensional motion may comprise more complex or alternatedesigns. For example, eight radially aligned active area sets disposedat 45 degree increments. Alternatively, three radially aligned activearea sets disposed at 120 degree increments may be suitable for 2D and3-D motion.

[0096] In addition, although transducer 180 is shown with only one setof axial active areas, the structure of FIG. 3C is modular. In otherwords, the four radially aligned active area sets disposed at 90 degreeincrements may occur multiple times in an axial direction. For example,radially aligned active area sets that allow two- and three-dimensionalmotion may be repeated ten times to provide a snake like roboticmanipulator with ten independently controllable links.

[0097] 3.3 Nested Rolled Electroactive Polymer Devices

[0098] Some applications desire an increased stroke from a rolledelectroactive polymer device. In one embodiment, a nested configurationis used to increase the stroke of an electroactive polymer device. In anested configuration, one or more electroactive polymer rolls are placedin the hollow central part of another electroactive polymer roll.

[0099] FIGS. 3E-G illustrate exemplary cross-sectional views of a nestedelectroactive polymer device 200, taken through the vertical midpoint ofthe cylindrical roll, in accordance with one embodiment of the presentinvention. Nested device 200 comprises three electroactive polymer rolls202, 204, and 206. Each polymer roll 202, 204, and 206 includes a singleactive area that provides uniform deflection for each roll. Electrodesfor each polymer roll 202, 204, and 206 may be electrically coupled toactuate (or produce electrical energy) in unison, or may be separatelywired for independent control and performance. The bottom ofelectroactive polymer roll 202 is connected to the top of the next outerelectroactive polymer roll, namely roll 204, using a connector 205.Connector 205 transfers forces and deflection from one polymer roll toanother. Connector 205 preferably does not restrict motion between therolls and may comprise a low friction and insulating material, such asteflon. Likewise, the bottom of electroactive polymer roll 204 isconnected to the top of the outermost electroactive polymer roll 206.The top of polymer roll 202 is connected to an output shaft 208 thatruns through the center of device 200. Although nested device 200 isshown with three concentric electroactive polymer rolls, it isunderstood that a nested device may comprise another number ofelectroactive polymer rolls.

[0100] Output shaft 208 may provide mechanical output for device 200 (ormechanical interface to external objects). Bearings may be disposed in abottom housing 212 and allow substantially frictionless linear motion ofshaft 208 axially through the center of device 200. Housing 212 is alsoattached to the bottom of roll 206 and includes bearings that allowtravel of shaft 208 through housing 212.

[0101] The deflection of shaft 208 comprises a cumulative deflection ofeach electroactive polymer roll included in nested device 200. Morespecifically, individual deflections of polymer roll 202, 204 and 206will sum to provide the total linear motion output of shaft 208. FIG. 3Eillustrates nested electroactive polymer device 200 with zerodeflection. In this case, each polymer roll 202, 204 and 206 is in anunactuated (rest) position and device 200 is completely contracted. FIG.3F illustrates nested electroactive polymer device 200 with 20% strainfor each polymer roll 202, 204 and 206. Thus, shaft 208 comprises a 60%overall strain relative to the individual length of each roll.Similarly, FIG. 3G illustrates nested electroactive polymer device 200with 50% strain for each polymer roll 202, 204 and 206. In this case,shaft 208 comprises a 150% overall strain relative to the individuallength of each roll. By nesting multiple electroactive polymer rollsinside each other, the strains of individual rolls add up and provide alarger net stroke than would be achieved using a single roll. Nestedelectroactive polymer rolled devices are then useful for applicationsrequiring large strains and compact packages.

[0102] In another embodiment, shaft 208 may be a shaft inside a tube,which allows the roll to expand and contract axially without bending inanother direction. While it would be advantageous in some situations tohave 208 attached to the top of 202 and running through bearings, shaft208 could also be two separate pieces: 1) a shaft connected to 212 andprotruding axially about ⅘ of the way toward the top of 206, and 2) atube connected to the top of 206 and protruding axially about ⅘ of theway toward 212, partially enveloping the shaft connected to 212.

[0103] FIGS. 3H-J illustrate exemplary vertical cross-sectional views ofa nested electroactive polymer device 220 in accordance with anotherembodiment of the present invention. Nested device 220 comprises threeelectroactive polymer rolls 222, 224, and 226. Each polymer roll 222,224, and 226 includes a single active area that provides uniformdeflection for each roll.

[0104] In this configuration, adjacent electroactive polymer rolls areconnected at their common unconnected end. More specifically, the bottomof electroactive polymer roll 222 is connected to the bottom of the nextouter electroactive polymer roll, namely roll 224. Likewise, the top ofelectroactive polymer roll 224 is connected to the top of the outermostelectroactive polymer roll 226. The top of polymer roll 222 is connectedto an output shaft 228 that runs through the center of device 220.Similar to as that described with respect to shaft 208, shaft 222 may bea shaft inside a tube, which allows the roll to expand and contractaxially without bending in another direction.

[0105]FIG. 3H shows the unactuated (rest) position of device 220. FIG.31 shows a contracted position of device 220 via actuation of polymerroll 224. FIG. 3J shows an extended position of device 220 via actuationof polymer rolls 222 and 226. In the unactuated (rest) position of FIG.3H, the shaft 208 position will be somewhere between the contractedposition of FIG. 31 and the extended position of FIG. 3J, depending onthe axial lengths of each individual roll.

[0106] This nested design may be repeated with an increasing number oflayers to provide increased deflection. Actuating every otherroll—starting from the first nested roll—causes shaft 228 to contract.Actuating every other roll—starting from the outermost roll—causes shaft228 to extend. One benefit to the design of nested device 220 is thatcharge may be shunted from one polymer roll to another, thus conservingoverall energy usage. It is worth noting that each device 200 or 220 maybe operated as a high strain generator or sensor that receivesmechanical energy via shaft 208 or 228, as will be described in furtherdetail below.

[0107] 3.4 Negative Spring Constant Designs

[0108] A mechanism of the present invention may vary the force itprovides with deflection of the transducer or device. For rolledelectroactive polymer devices that employ a spring, as the deviceaxially extends, the output force of the device typically decreases as aresult of the spring. In many applications, it is desirable to implementmechanical input, such as a linear actuator, whose output force isconstant over the range of deflection—or increases withextension—according to the needs of the application. Such constant ornegative spring constant mechanical input may be achieved using severalmechanisms. Indeed, many such mechanisms could be attached to a rolledactuator externally or within the hollow interior. Additionally, it isalso possible to make the spring structure itself operate as a constantor negative spring constant spring. For example, the spring could bemade by stacking several Belleville washers on a rigid rod with a flangeat one to restrain the most proximal Belleville washer. Bellevillewashers are circular disks with a central hole that are slightlyconical. When compressed with sufficient force they can be made to passthrough the point at which they are completely flat and become conicalin the opposite direction from the original configuration. This flangeis attached to one end of the roll. At the other end of the stack is aflat washer that is attached to the other end of the roll that restrainsand allows the rolled polymer material to compress the Bellevillewashers past the point at which they become unstable and exert a forceto invert the orientation of the cone. Many other negative springconstant mechanisms could be used that do not require Bellevillewashers. These mechanism need only be placed between each washer in astack of flat washers so that the entire stack behaves as a negativeconstant spring. In all these examples, the edges of the washers providesupport for the stretched polymer film. Other constant or negativespring constant mechanical input and there use to enhance the output ofdielectric elastomer actuators are further described in patent Ser. No.09/779,373, which is incorporated herein for all purposes.

[0109]FIG. 3K illustrates a rolled electroactive polymer device 240 thatallows a designer to vary the deflection vs. force profile of thedevice. Device 240 comprises end pieces 242 and 244, rolledelectroactive polymer 246, spring 248, rod 250, magnet 252,ferromagnetic core 254, and a magnet apparatus 256.

[0110] End pieces 242 and 244, rolled electroactive polymer 246, andspring 248 are similar in structure and function as that described abovewith respect to FIG. 2A. Rod 250 is coupled to end pieces 244 and slideswithin end piece 244. In one embodiment, the rod 250 is a solid rod thatextends in length from bottom end piece 242 to top end piece 244, andscrews into end piece 244 using mating threads in rod 250 and end piece244. In one embodiment, the entire rod 250 is made of 2 pieces: 1) a rodwith different diameters along the length of the rod (according to theembodiment shown in FIG. 3K, it would have four different diameters),and screwed into end piece 244 and 2) a tapered ferromagnetic core witha cylindrical hole of the same diameter as the top of rod 250. Thus, rod250 is fixed to end piece 244, and slides relative to end piece 242 andmagnet apparatus 256 as the polymer expands and contracts. Ferromagneticcore 254 is disposed on rod 250 somewhere between end pieces 242 and244. Ferromagnetic core 254 is a metal (e.g., steel) or similar materialthat provides magnetic attraction and forces between itself and amagnetic field. Connected rigidly to top end piece 242 is magneticapparatus 246, which supports and aligns a ring shaped magnet 252.Magnet 252 is thereby disposed concentrically with rod 250 andferromagnetic core 254. Magnet 252 produces a magnetic field thatattracts core 254.

[0111] Magnet 252 has a taper on its inner edge 253; and core 254 has acorresponding taper on its outer edge 255. With changing polymer 246deflection and motion of rod 250, magnet 252 is drawn closer to core254—thus exerting a force on slide 250 that increases as magnet 252nears core 254. In one embodiment, magnet 252 is magnetized radially.

[0112] Thus, as rod 250 extends, the output force of slide 250 due tospring 248 gets weaker, but the output force of rod 250 due to theinternal magnetic assembly gets stronger. Spring 248 and the internalmagnetic assembly may be designed or configured to attain a desiredforce relationship with deflection. For example, spring 248 and internalmagnetic assembly may be designed and configured such that the net forceof rod 250 increases with polymer 246 deflection.

[0113] Since the force of magnetic attraction between magnet 252 andcore 254 decreases with the square of rod 250 deflection, designing andimplementing a 1:1 correspondence between linear slide deflection andmagnetic attraction would result in a narrow operating range. However,with the tapered magnet design of device 240, as rod 250 encounterslarge deflections, the force of magnetic attraction changes onlyslightly, thereby resulting in a larger deflection operating range.

[0114] In another embodiment, magnet 252 has a vertical inner edge (acylinder with a straight hole through it) and core 254 also has amatching cylindrical outer profile. In this case, there is a force frommagnet 252 pulling core 254 completely inside magnet 252. As magnet 252nears core 254, it similarly draws core 254 into the magnetic cylinder,thus resulting in a net force on rod 250. This second configurationallows simpler manufacture.

[0115] In one embodiment, device 240 also comprises a hard stop 258attached to end piece 242. Hard stop 258 places a physical limit on howclose magnet 252 can get to core 254, and prevents contact betweenmagnet 252 and core 254. Alternatively, a barrier layer may be disposedbetween magnet 252 and core 254, such as a layer of plastic, cardboard,foam, etc., to prevent metal on magnet contact.

[0116] 4. Multifunctionality

[0117] Electroactive polymers have many functional uses. In addition toactuation, active areas of the present invention may also be used forgeneration and production of electrical energy, sensing, stiffnesscontrol, or damping control.

[0118]FIGS. 1A and 1B may be used to show one manner in which thetransducer portion 10 converts mechanical energy to electrical energy.For example, if the transducer portion 10 is mechanically stretched byexternal forces to a thinner, larger area shape such as that shown inFIG. 1B, and a relatively small voltage difference (less than thatnecessary to actuate the film to the configuration in FIG. 1B) isapplied between electrodes 14 and 16, the transducer portion 10 willcontract in area between the electrodes to a shape such as in FIG. 1Awhen the external forces are removed. Stretching the transducer refersto deflecting the transducer from its original restingposition—typically to result in a larger net area between theelectrodes, e.g. in the plane defined by directions 18 and 20 betweenthe electrodes. The resting position refers to the position of thetransducer portion 10 having no external electrical or mechanical inputand may comprise any pre-strain in the polymer. Once the transducerportion 10 is stretched, the relatively small voltage difference isprovided such that the resulting electrostatic forces are insufficientto balance the elastic restoring forces of the stretch. The transducerportion 10 therefore contracts, and it becomes thicker and has a smallerplanar area in the plane defined by directions 18 and 20 (orthogonal tothe thickness between electrodes). When polymer 12 becomes thicker, itseparates electrodes 14 and 16 and their corresponding unlike charges,thus raising the electrical energy and voltage of the charge. Further,when electrodes 14 and 16 contract to a smaller area, like chargeswithin each electrode compress, also raising the electrical energy andvoltage of the charge. Thus, with different charges on electrodes 14 and16, contraction from a shape such as that shown in FIG. 1B to one suchas that shown in FIG. 1A raises the electrical energy of the charge.That is, mechanical deflection is being turned into electrical energyand the transducer portion 10 is acting as a ‘generator’.

[0119] When a relatively small voltage difference is applied betweenelectrodes 14 and 16, deflection of transducer portion 10 will tend tochange the voltage difference between the electrodes or drive charge toor from the electrodes, or do both, depending on the electrical stateimposed on the electrodes 14 and 16. As polymer 12 changes in size, thechanging electrical properties and voltage may be detected, dissipated,and/or used. For example, the change in voltage difference between theelectrodes may be used to drive current to or from one of the electrodeswhich is dissipated through a resistor.

[0120] Some or all of the charge and energy can be removed when thetransducer portion 10 is fully contracted in the plane defined bydirections 18 and 20. Alternatively, some or all of the charge andenergy can be removed during contraction. If the electric field pressurein the polymer increases and reaches balance with the mechanical elasticrestoring forces and external load during contraction, the contractionwill stop before full contraction, and no further elastic mechanicalenergy will be converted to electrical energy. Removing some of thecharge and stored electrical energy reduces the electrical fieldpressure, thereby allowing contraction to continue. The exact electricalbehavior of the transducer portion 10 when operating in generator modedepends on any electrical and mechanical loading as well as theintrinsic properties of polymer 12 and electrodes 14 and 16.

[0121] In some cases, the transducer portion 10 may be describedelectrically as a variable capacitor. The capacitance decreases for theshape change going from that shown in FIG. 1B to that shown in FIG. 1A.Typically, the voltage difference between electrodes 14 and 16 will beraised by contraction. This is normally the case, for example, ifadditional charge is not added or subtracted from electrodes 14 and 16during the contraction process. The increase in electrical energy, U,may be illustrated by the formula U=0.5 Q²/C, where Q is the amount ofpositive charge on the positive electrode and C is the variablecapacitance which relates to the intrinsic dielectric properties ofpolymer 12 and its geometry. If Q is fixed and C decreases, then theelectrical energy U increases. The increase in electrical energy andvoltage can be recovered or used in a suitable device or electroniccircuit in electrical communication with electrodes 14 and 16. Inaddition, the transducer portion 10 may be mechanically coupled to amechanical input that deflects the polymer and provides mechanicalenergy.

[0122] For a transducer having a substantially constant thickness, onemechanism for differentiating the performance of the transducer, or aportion of the transducer associated with a single active area, as beingan actuator or a generator is in the change in net area orthogonal tothe thickness associated with the polymer deflection. For thesetransducers or active areas, when the deflection causes the net area ofthe transducer/active area to decrease and there is charge on theelectrodes, the transducer/active area is converting from mechanical toelectrical energy and acting as a generator. Conversely, when thedeflection causes the net area of the transducer/active area to increaseand charge is on the electrodes, the transducer/active area isconverting electrical to mechanical energy and acting as an actuator.The change in area in both cases corresponds to a reverse change in filmthickness, i.e. the thickness contracts when the planar area expands,and the thickness expands when the planar area contracts. Both thechange in area and change in thickness determine the amount of energythat is converted between electrical and mechanical. Since the effectsdue to a change in area and corresponding change in thickness arecomplementary, only the change in area will be discussed herein for sakeof brevity. In addition, although deflection of an electroactive polymerwill primarily be discussed as a net increase in area of the polymerwhen the polymer is being used in an actuator to produce mechanicalenergy, it is understood that in some cases (i.e. depending on theloading), the net area may decrease to produce mechanical work. Thus,devices of the present invention may include both actuator and generatormodes, depending on how the polymer is arranged and applied.

[0123] Electroactive polymers of the present invention may also beconfigured as a sensor. Generally, electroactive polymer sensors of thisinvention detect a “parameter” and/or changes in the parameter. Theparameter is usually a physical property of an object such as itstemperature, density, strain, deformation, velocity, location, contact,acceleration, vibration, volume, pressure, mass, opacity, concentration,chemical state, conductivity, magnetization, dielectric constant, size,etc. In some cases, the parameter being sensed is associated with aphysical “event”. The physical event that is detected may be theattainment of a particular value or state of a physical or chemicalproperty.

[0124] An electroactive polymer sensor is configured such that a portionof the electroactive polymer deflects in response to the change in aparameter being sensed. The electrical energy state and deflection stateof the polymer are related. The change in electrical energy or a changein the electrical impedance of an active area resulting from thedeflection may then be detected by sensing electronics in electricalcommunication with the active area electrodes. This change may comprisea capacitance change of the polymer, a resistance change of the polymer,and/or resistance change of the electrodes, or a combination thereof.Electronic circuits in electrical communication with electrodes detectthe electrical property change. If a change in capacitance or resistanceof the transducer is being measured for example, one applies electricalenergy to electrodes included in the transducer and observes a change inthe electrical parameters.

[0125] In one embodiment, deflection is input into an active area sensorin some manner via one or more coupling mechanisms. In one embodiment,the changing property or parameter being measured by the sensorcorresponds to a changing property of the electroactive polymer, e.g.displacement or size changes in the polymer, and no coupling mechanismis used. Sensing electronics in electrical communication with theelectrodes detect change output by the active area. In some cases, alogic device in electrical communication with sensing electronics ofsensor quantifies the electrical change to provide a digital or othermeasure of the changing parameter being sensed. For example, the logicdevice may be a single chip computer or microprocessor that processesinformation produced by sensing electronics. Electroactive polymersensors are further described in Ser. No. 10/007,705, which isincorporated herein by reference for all purposes.

[0126] An active area may be configured such that sensing is performedsimultaneously with actuation of the active area. For a monolithictransducer, one active area may be responsible for actuation and anotherfor sensing. Alternatively, the same active area of a polymer may beresponsible for actuation and sensing. In this case, a low amplitude,high frequency AC (sensing) signal may be superimposed on the driving(actuation) signal. For example, a 1000 Hz sensing signal may besuperimposed on a 10 Hz actuation signal. The driving signal will dependon the application, or how fast the actuator is moving, but drivingsignals in the range from less than 0.1 Hz to about 1 million Hz aresuitable for many applications. In one embodiment, the sensing signal isat least about 10 times faster than the motion being measured. Sensingelectronics may then detect and measure the high frequency response ofthe polymer to allow sensor performance that does not interfere withpolymer actuation. Similarly, if impedance changes are detected andmeasured while the electroactive polymer transducer is being used as agenerator, a small, high-frequency AC signal may be superimposed on thelower-frequency generation voltage signal. Filtering techniques may thenseparate the measurement and power signals.

[0127] Active areas of the present invention may also be configured toprovide variable stiffness and damping functions. In one embodiment,open loop techniques are used to control stiffness and/or damping of adevice employing an electroactive polymer transducer; thereby providingsimple designs that deliver a desired stiffness and/or dampingperformance without sensor feedback. For example, control electronics inelectrical communication with electrodes of the transducer may supply asubstantially constant charge to the electrodes. Alternately, thecontrol electronics may supply a substantially constant voltage to theelectrodes. Systems employing an electroactive polymer transducer offerseveral techniques for providing stiffness and/or damping control. Anexemplary circuit providing stiffness/damping control is provided below.

[0128] While not described in detail, it is important to note thatactive areas and transducers in all the figures and discussions for thepresent invention may convert between electrical energy and mechanicalenergy bi-directionally (with suitable electronics). Thus, any of therolled polymers, active areas, polymer configurations, transducers, anddevices described herein may be a transducer for converting mechanicalenergy to electrical energy (generation, variable stiffness or damping,or sensing) and for converting electrical energy to mechanical energy(actuation, variable stiffness or damping, or sensing). Typically, agenerator or sensor active area of the present invention comprises apolymer arranged in a manner that causes a change in electric field inresponse to deflection of a portion of the polymer. The change inelectric field, along with changes in the polymer dimension in thedirection of the field, produces a change in voltage, and hence a changein electrical energy.

[0129] Often the transducer is employed within a device that comprisesother structural and/or functional elements. For example, externalmechanical energy may be input into the transducer in some manner viaone or more mechanical transmission coupling mechanisms. For example,the transmission mechanism may be designed or configured to receivebiologically-generated mechanical energy and to transfer a portion ofthe biologically-generated mechanical energy to a portion of a polymerwhere the transferred portion of the biologically generated mechanicalenergy results in a deflection in the transducer. Thebiologically-generated mechanical energy may produce an inertial forceor a direct force where a portion of the inertial force or a portion ofthe direct force is received by the transmission mechanism. In oneembodiment, the direct force may be from a foot strike.

[0130] 5. Conditioning Electronics

[0131] Devices of the present invention may also rely on conditioningelectronics that provide or receive electrical energy from electrodes ofan active area for one of the electroactive polymer functions mentionedabove. Conditioning electronics in electrical communication with one ormore active areas may include functions such as stiffness control,energy dissipation, electrical energy generation, polymer actuation,polymer deflection sensing, control logic, etc.

[0132] For actuation, electronic drivers may be connected to theelectrodes. The voltage provided to electrodes of an active area willdepend upon specifics of an application. In one embodiment, an activearea of the present invention is driven electrically by modulating anapplied voltage about a DC bias voltage. Modulation about a bias voltageallows for improved sensitivity and linearity of the transducer to theapplied voltage. For example, a transducer used in an audio applicationmay be driven by a signal of up to 200 to 100 volts peak to peak on topof a bias voltage ranging from about 750 to 2000 volts DC.

[0133] Suitable actuation voltages for electroactive polymers, orportions thereof, may vary based on the material properties of theelectroactive polymer, such as the dielectric constant, as well as thedimensions of the polymer, such as the thickness of the polymer film Forexample, actuation electric fields used to actuate polymer 12 in FIG. 2Amay range in magnitude from about 0 V/m to about 440 MV/m. Actuationelectric fields in this range may produce a pressure in the range ofabout 0 Pa to about 10 MPa. In order for the transducer to producegreater forces, the thickness of the polymer layer may be increased.Actuation voltages for a particular polymer may be reduced by increasingthe dielectric constant, decreasing the polymer thickness, anddecreasing the modulus of elasticity, for example.

[0134]FIG. 4 illustrates an electrical schematic of an open loopvariable stiffness/damping system in accordance with one embodiment ofthe present invention. System 130 comprises an electroactive polymertransducer 132, voltage source 134, control electronics comprisingvariable stiffness/damping circuitry 136 and open loop control 138, andbuffer capacitor 140.

[0135] Voltage source 134 provides the voltage used in system 130. Inthis case, voltage source 134 sets the minimum voltage for transducer132. Adjusting this minimum voltage, together with open loop control138, adjusts the stiffness provided by transducer 132. Voltage source134 also supplies charge to system 130. Voltage source 134 may include acommercially available voltage supply, such as a low-voltage batterythat supplies a voltage in the range of about 1-15 Volts, and step-upcircuitry that raises the voltage of the battery. In this case, voltagestep-down performed by step-down circuitry in electrical communicationwith the electrodes of transducer 132 may be used to adjust anelectrical output voltage from transducer 132. Alternately, voltagesource 134 may include a variable step-up circuit that can produce avariable high voltage output from the battery. As will be described infurther detail below, voltage source 134 may be used to apply athreshold electric field as described below to operate the polymer in aparticular stiffness regime.

[0136] The desired stiffness or damping for system 130 is controlled byvariable stiffness/damping circuitry 136, which sets and changes anelectrical state provided by control electronics in system 130 toprovide the desired stiffness/damping applied by transducer 132. In thiscase, stiffness/damping circuitry 36 inputs a desired voltage to voltagesource 134 and/or inputs a parameter to open loop control 138.Alternately, if step-up circuitry is used to raise the voltage source134, circuitry 136 may input a signal to the step-up circuitry to permitvoltage control.

[0137] As transducer 132 deflects, its changing voltage causes charge tomove between transducer 132 and buffer capacitor 140. Thus, externallyinduced expansion and contraction of transducer 132, e.g., from avibrating mechanical interface, causes charge to flow back and forthbetween transducer 132 and buffer capacitor 140 through open loopcontrol 138. The rate and amount of charge moved to or from transducer132 depends on the properties of buffer capacitor 140, the voltageapplied to transducer 132, any additional electrical components in theelectrical circuit (such as a resistor used as open loop control 138 toprovide damping functionality as current passes therethrough), themechanical configuration of transducer 132, and the forces applied to orby transducer 132. In one embodiment, buffer capacitor 140 has a voltagesubstantially equal to that of transducer 132 for zero displacement oftransducer 132, the voltage of system 130 is set by voltage source 134,and open loop control 138 is a wire; resulting in substantially freeflow of charge between transducer 132 and buffer capacitor 140 fordeflection of transducer 132.

[0138] Open loop control 138 provides a passive (no external energysupplied) dynamic response for stiffness applied by transducer 132.Namely, the stiffness provided by transducer 132 may be set by theelectrical components included in system 130, such as the controlelectronics and voltage source 134, or by a signal from controlcircuitry 136 acting upon one of the electrical components. Either way,the response of transducer 132 is passive to the external mechanicaldeflections imposed on it. In one embodiment, open loop control 138 is aresistor. One can also set the resistance of the resistor to provide anRC time constant relative to a time of interest, e.g., a period ofoscillation in the mechanical system that the transducer is implementedin. In one embodiment, the resistor has a high resistance such that theRC time constant of open loop control 138 and transducer 132 connectedin series is long compared to a frequency of interest. In this case, thetransducer 132 has a substantially constant charge during the time ofinterest. A resistance that produces an RC time constant for theresistor and the transducer in the range of about 5 to about 30 timesthe period of a frequency of interest may be suitable for someapplications. For applications including cyclic motion, increasing theRC time constant much greater than the mechanical periods of interestallows the amount of charge on electrodes of transducer 132 to remainsubstantially constant during one cycle. In cases where the transduceris used for damping, a resistance that produces an RC time constant forthe resistor and the transducer in the range of about 0.1 to about 4times the period of a frequency of interest may be suitable. As one ofskill in the art will appreciate, resistances used for the resistor mayvary based on application, particularly with respect to the frequency ofinterest and the size (and therefore capacitance C) of the transducer132.

[0139] In one embodiment of a suitable electrical state used to controlstiffness and/or damping using open loop techniques, the controlelectronics apply a substantially constant charge to electrodes oftransducer 132, aside from any electrical imperfections or circuitdetails that minimally affect current flow. The substantially constantcharge results in an increased stiffness for the polymer that resistsdeflection of transducer 132. One electrical configuration suitable forachieving substantially constant charge is one that has a high RC timeconstant, as described. When the value of the RC time constant of openloop control 138 and transducer 132 is long compared to the frequency ofinterest, the charge on the electrodes for transducer 132 issubstantially constant. Further description of stiffness and/or dampingcontrol is further described in commonly owned patent application Ser.No. 10/053,511, which is described herein for all purposes.

[0140] For generation, mechanical energy may be applied to the polymeror active area in a manner that allows electrical energy changes to beremoved from electrodes in contact with the polymer. Many methods forapplying mechanical energy and removing an electrical energy change fromthe active area are possible. Rolled devices may be designed thatutilize one or more of these methods to receive an electrical energychange. For generation and sensing, the generation and utilization ofelectrical energy may require conditioning electronics of some type. Forinstance, at the very least, a minimum amount of circuitry is needed toremove electrical energy from the active area. Further, as anotherexample, circuitry of varying degrees of complexity may be used toincrease the efficiency or quantity of electrical generation in aparticular active area or to convert an output voltage to a more usefulvalue.

[0141]FIG. 5A is block diagram of one or more active areas 600 on arolled transducer that connected to power conditioning electronics 610.Potential functions that may be performed by the power conditioningelectronics 610 include but are not limited to 1) voltage step-upperformed by step-up circuitry 602, which may be used when applying avoltage to active areas 600, 2) charge control performed by the chargecontrol circuitry 604 which may be used to add or to remove charge fromthe active areas 600 at certain times, 3) voltage step-down performed bythe step-down circuitry 608 which may be used to adjust an electricaloutput voltage to a transducer. All of these functions may not berequired in the conditioning electronics 610. For instance, sometransducer devices may not use step-up circuitry 602, other transducerdevices may not use step-down circuitry 608, or some transducer devicesmay not use step-up circuitry and step-down circuitry. Also, some of thecircuit functions may be integrated. For instance, one integratedcircuit may perform the functions of both the step-up circuitry 602 andthe charge control circuitry 608.

[0142]FIG. 5B is a circuit schematic of an rolled device 603 employing atransducer 600 for one embodiment of the present invention. As describedabove, transducers of the present invention may behave electrically asvariable capacitors. To understand the operation of the rolledtransducer 603, operational parameters of the rolled transducer 603 attwo times, t₁ and t₂ may be compared. Without wishing to be constrainedby any particular theory, a number of theoretical relationshipsregarding the electrical performance the generator 603 are developed.These relationships are not meant in any manner to limit the manner inwhich the described devices are operated and are provided forillustrative purposes only.

[0143] At a first time, t₁, rolled transducer 600 may possess acapacitance, C₁, and the voltage across the transducer 600 may bevoltage 601, V_(B). The voltage 601, V_(B), may be provided by thestep-up circuitry 602. At a second time t₂, later than time t₁, therolled transducer 600 may posses a capacitance C₂ which is lower thanthe capacitance C1. Generally speaking, the higher capacitance C1 occurswhen the polymer transducer 600 is stretched in area, and the lowercapacitance C2 occurs when the polymer transducer 600 is contracted orrelaxed in area. Without wishing to bound by a particular theory, thechange in capacitance of a polymer film with electrodes may be estimatedby well known formulas relating the capacitance to the film's area,thickness, and dielectric constant.

[0144] The decrease in capacitance of the rolled transducer 600 betweent₁ and t₂ will increase the voltage across the rolled transducer 600.The increased voltage may be used to drive current through diode 616.The diode 615 may be used to prevent charge from flowing back into thestep-up circuitry at such time. The two diodes, 615 and 616, function ascharge control circuitry 604 for rolled transducer 600 which is part ofthe power conditioning electronics 610 (see FIG. 5A). More complexcharge control circuits may be developed depending on the configurationof the generator 603 and the one or more transducers 600 and are notlimited to the design in FIG. 5B.

[0145] A rolled transducer may also be used as an electroactive polymersensor to measure a change in a parameter of an object being sensed.Typically, the parameter change induces deflection in the transducer,which is converted to an electrical change output by electrodes attachedto the transducer. Many methods for applying mechanical or electricalenergy to deflect the polymer are possible. Typically, the sensing ofelectrical energy from a transducer uses electronics of some type. Forinstance, a minimum amount of circuitry is needed to detect a change inthe electrical state across the electrodes.

[0146]FIG. 7 is a schematic of a sensor 350 employing a rolledtransducer 351 according to one embodiment of the present invention. Asshown in FIG. 7, sensor 350 comprises rolled transducer 351 and variouselectronics 355 in electrical communication with the electrodes includedin the transducer 351. Electronics 355 are designed or configured toadd, remove, and/or detect electrical energy from rolled transducer 351.While many of the elements of electronics 355 are described as discreteunits, it is understood that some of the circuit functions may beintegrated. For instance, one integrated circuit may perform thefunctions of both the logic device 365 and the charge control circuitry357.

[0147] In one embodiment, the rolled transducer 351 is prepared forsensing by initially applying a voltage between its electrodes. In thiscase, a voltage, V_(I), is provided by the voltage 352. Generally, V_(I)is less than the voltage required to actuate rolled transducer 351. Insome embodiments, a low-voltage battery may supply voltage, V_(I), inthe range of about 1-15 Volts. In any particular embodiment, choice ofthe voltage, V_(I) may depend on a number of factors such as the polymerdielectric constant, the size of the polymer, the polymer thickness,environmental noise and electromagnetic interference, compatibility withelectronic circuits that might use or process the sensor information,etc. The initial charge is placed on rolled transducer 351 usingelectronics control sub-circuit 357. The electronics control sub-circuit357 may typically include a logic device such as single chip computer ormicrocontroller to perform voltage and/or charge control functions onrolled transducer 351. The electronics control sub-circuit 357 is thenresponsible for altering the voltage provided by voltage 352 toinitially apply the relatively low voltage on rolled transducer 351.

[0148] Sensing electronics 360 are in electrical communication with theelectrodes of rolled transducer 351 and detect the change in electricalenergy or characteristics of rolled transducer 351. In addition todetection, sensing electronics 360 may include circuits configured todetect, measure, process, propagate, and/or record the change inelectrical energy or characteristics of rolled transducer 351.Electroactive polymer transducers of the present invention may behaveelectrically in several ways in response to deflection of theelectroactive polymer transducer. Correspondingly, numerous simpleelectrical measurement circuits and systems may be implemented withinsensing electronics 360 to detect a change in electrical energy ofrolled transducer 351. For example, if rolled transducer 351 operates incapacitance mode, then a simple capacitance bridge may be used to detectchanges in rolled transducer 351 capacitance. In another embodiment, ahigh resistance resistor is disposed in series with rolled transducer351 and the voltage drop across the high resistance resistor is measuredas the rolled transducer 351 deflects. More specifically, changes inrolled transducer 351 voltage induced by deflection of the electroactivepolymer are used to drive current across the high resistance resistor.The polarity of the voltage change across resistor then determines thedirection of current flow and whether the polymer is expanding orcontracting. Resistance sensing techniques may also be used to measurechanges in resistance of the polymer included or changes in resistanceof the electrodes. Some examples of these techniques are described incommonly owned patent application Ser. No. 10/007,705, which waspreviously incorporated by reference.

[0149] 6. Fabrication

[0150] One advantage of rolled electro active polymers of the presentinvention is simplified manufacture to obtain multilayer electroactivepolymer devices. FIGS. 6A-6D describe the manufacture of a rolledelectroactive polymer device in accordance with one embodiment of thepresent invention. While not described in detail, it is understood thatfabrication techniques described below may be manually implemented,automated, or may comprise a combination of manual and automatedtechniques.

[0151] Fabrication according to one embodiment of the present inventionemploys a frame or fixture to facilitate rolling of an electro activepolymer. FIG. 6B illustrates a rolling fixture 650 useful forfacilitating the rolling of one or more electro active polymers. Fixture650 includes length 652 and width 654 dimensioned according to thedesired unrolled circumferential length and rolled height, respectively,of an electroactive polymer to be rolled. Smaller electroactive polymersmay be fashioned using fixture 650 by using a portion of the fixture.For example, a rolled electroactive polymer may have an unrolledcircumferential length less than height 652. In many cases, theelectroactive polymer to be rolled is initially smaller than the rollingdimensions of fixture 650 and pre strain is used to increase the size ofthe polymer (see FIG. 6C).

[0152] Rolling fixture 650 fixtures an electroactive polymer duringrolling, which in this context refers to one or more of: a) dimensioningan electroactive polymer for subsequent rolling, b) establishing andmaintaining a desired prestrain level including holding theelectroactive polymer and overcoming any elastic restoring forces in thepolymer resulting from prestrain stretching, and c) functional receptionof the rolling mechanism or process. Rolling fixture 650 may include anyfeatures or structures that provide or facilitate one of thesefunctions. For example, to minimize bubbles and other defects betweenpolymer layers during rolling, surface 656 is preferably substantiallysmooth with no surface defects that may introduce bumps or otherinconsistencies in the surface of the polymer during rolling.

[0153] For some acrylic electroactive polymers, such as the such as VHB4910 acrylic elastomer mentioned above for example, the acrylic has ahigh adhesion and may adhere to surface 656, thereby complicating therolling process. Surface 656 allows the polymer to be rolled withoutcomplications. In one embodiment to overcome adhesive complications,surface 656 comprises a Teflon coating. In another embodiment in whichfixture 650 is made from a rigid acrylic or when the electroactivepolymer does not have adhesive properties, a tape or other adhesivecontrol layer may be applied to the surface 656 to achieve a desiredadhesiveness between the polymer and fixture rolling surface. Theadhesive control layer eases peeling off of an adhesive polymer duringrolling. In a specific embodiment, a crystal clear tape such as Scotchbrand Crystal Clear Tape as provided by 3M Company of St. Paul, Minn. isused as an adhesive control layer.

[0154] When prestrain is applied to the polymer before rolling,receiving surface 656 preferably provides sufficient adhesion such thatthe prestrain is maintained by adhesion between the polymer and surface656. However, as mentioned above, surface 656 is not so adhesive as torestrict peeling off of the polymer during the rolling process. Thiscreates an adhesion range for the interface between surface 656 and thepolymer that depends on the adhesion properties between theelectroactive polymer and rolling surface 656. Thus, selection of arolling surface 656 or an additional adhesive control layer may be usedto control the interface between surface 656 and the polymer.

[0155] Fabrication according to the present invention may also rely onone or more additional fabrication fixtures or devices. Often, prestrainis applied to an electroactive polymer. This involves stretching polymermaterial, such as a thin film, from an area initially smaller thanrolling dimensions to an area close to the rolling dimensions; andimplies that the polymer must be held that this larger size duringrolling. FIG. 6C illustrates a stretching fixture 660 useful forstretching an electroactive polymer and maintaining prestrain inaccordance with one embodiment of the present invention. Stretchingfixture 660 includes a substantially flat rigid frame 662 that defines acentral opening or hole 664. A polymer 668 is stretched in bothdirections 667 and 669 and adhered to frame 662 perimetrically aroundhole 664. The adhesion between polymer 668 and frame 662 will depend onelectroactive polymer material and frame 662 material. For example,electroactive polymer 668 may be an acrylic polymer with adhesiveproperties and frame 662 may be a rigid acrylic plate that providessignificant adhesion to an acrylic electroactive polymer with adhesiveproperties. In other cases, securing and adhesive mechanisms such asremovable clamps and two way tape may be applied perimetrically, or inportions, about hole 664 to hold the prestrain polymer 668 to frame 660in a desired state of prestrain.

[0156]FIG. 6A illustrates a process flow 640 for fabricatingelectroactive polymer device comprising a rolled electroactive polymerin accordance with one embodiment of the present invention. Methods inaccordance with the present invention may include up to severaladditional steps not described or illustrated herein order not toobscure the present invention.

[0157] Process flow 640 begins by receiving an electroactive polymer(641). The polymer may be a commercially available product such as acommercially available acrylic elastomer film. Alternatively, thepolymer may be a film produced by one of casting, dipping, spin coatingor spraying. Spin coating typically involves applying a polymer mixtureon a rigid substrate and spinning to a desired thickness. The polymermixture may include the polymer, a curing agent and a volatiledispersant or solvent. The amount of dispersant, the volatility of thedispersant, and the spin speed may be altered to produce a desiredpolymer. By way of example, polyurethane films may be spin coated in asolution of polyurethane and tetrahydrofuran (THF) or cyclohexanone. Inthe case of silicon substrates, the polymer may be spin coated on analuminized plastic or a silicon carbide. The aluminum and siliconcarbide form a sacrificial layer that is subsequently removed by asuitable etchant. Films in the range of one micrometer thick may beproduced by spin coating in this manner. Spin coating of polymer films,such as silicone, may be done on a smooth non-sticking plasticsubstrate, such as polymethyl methacrylate or teflon. The polymer filmmay then be released by mechanically peeling or with the assistance ofalcohol or other suitable release agent.

[0158] In one embodiment, prestrain is applied to the polymer, beforerolling, by stretching the polymer in one or more directions (642). Asdescribed above, the prestrain may be anisotropic or isotropic. In oneembodiment, prestrain is applied by stretching the polymer from about300% to about 500% in direction 669 and 50% to about 200% in direction667 as shown in FIG. 6C. In a specific embodiment, prestrain is appliedby stretching the polymer 400% in direction 669 and 100% in direction667. Maintaining prestrain includes temporarily fixing the polymer insome manner. This may include use of a stretching fixture, such asfixture 660.

[0159] Electrodes are then patterned onto opposing surfaces of theelectroactive polymer (643). Specific techniques used to pattern theelectrodes will depend on the electrode type. For carbon greaseelectrodes, the carbon grease may be manually brushed onto the polymerwithin a brush. A stencil or template may be placed over the polymer tohelp define an electrode area during the brushing process. Carbon fibrilelectrodes may also be sprayed onto the polymer within a region definedby a stencil. In this case, a 0.1% dispersion of BN-type fibrils inethyl acetate may be suitable. Time may also be provided for sprayedelectrodes to dry. From about one hour to about eight hours may besuitable in some cases, depending on the composition and amount ofelectrode applied. In one embodiment, a stencil defines an electrodearea region of about 30 cm to about 35 cm by about 2 cm to about 5 cm.FIG. 6E illustrates a substantially rectangular electrode 673 patternedon the facing side of an electroactive polymer held by stretching frame660 in accordance with one embodiment of the present invention.

[0160] Both starting and finishing ends of polymer 668 include a portion671 and 677, respectively, that to do not include electrode material.Portion 677 is not electroded to allow an outermost layer for thefinished rolled device that does not include electrodes and acts as abarrier layer for mechanical protection and electrical isolation. Whenpolymer 668 is rolled about a metal spring, portion 671 is notelectroded to provide electrical isolation between inner layerelectrodes at the metal spring.

[0161] Leads may also be disposed in electrical communication with theelectrodes. When contact electrodes are used on both sides of polymer668, a lead is attached to each contact electrode on both sides ofpolymer 668. In a specific embodiment, the lead comprises one or morecopper or gold wires placed between aluminum foil and double sided tape.As shown in FIG. 6E, aluminum foil 672 is disposed along the top edge679 of electrode 673 on the facing side of polymer 668. Aluminum foil672 improves charge communication (charge distribution or collection)between electrode 673 and lead 675. Aluminum foil 672 is disposed alongthe edge of electrode 673, such that when the polymer is rolled, thealuminum foil 672 is proximate to either the top or bottom cylindricalend of the rolled device. A lead and aluminum foil are also disposedalong the bottom edge of an electrode patterned on the opposite side ofpolymer 668. For an acrylic electroactive polymer with adhesiveproperties, the aluminum foil may include a portion that overlaps thetop edge of the electrode and onto the adhesive polymer outside theelectrode. This second portion then adheres to the adhesive polymer viathe adhesive properties of the polymer. Lead 675 is disposed on aluminumfoil 672 and secured to aluminum foil 672 using two sided tape placedover top of both lead 675 and aluminum foil 672. In one embodiment, lead675 is a wire. The two sided tape secures lead 675 in position, and alsoprevents any sharp edges on lead 675 from damaging adjacent polymerlayers after rolling. Off-the-shelf aluminum foil and two sided tapesuch as 3M two sided tape may be suitable for use as aluminum foil 672and two sided tape 675, respectively.

[0162] In one embodiment, a multiple layer rolled construction is used(644). In this case, a second layer of electroactive polymer 680 isdisposed on top of the electrode polymer 668 (see FIG. 6F). As mentionedbefore, the layers in the multiple layer stack need not be the samematerial. Other types of polymer (electroactive polymer ornon-electroactive polymer) may be included in the stack, for example, tovary the stiffness of the stack.

[0163] For some acrylic electroactive polymers, adhesive properties ofthe acrylic polymer hold the layers together. In one embodiment,electrodes are not patterned for the second polymer 680. After rolling,electrode 673 on the top surface of polymer 668 acts as an electrode forboth the top side of polymer 668 and the bottom side of polymer 680.After rolling, the electrode 681 on the bottom side of polymer 668contacts the top side of polymer 680 and acts as an electrode for thetop side of polymer 680. Thus, after rolling, polymer 680 includeselectrodes that contact both planar surfaces. Another electroded polymerlayer may be disposed on top of polymer 680, along with another polymerlayer having no electrodes. All four may then be rolled. Thiseven-numbered layer construction in which one polymer is electroded andthe other is not may be repeated to produce 6 or 8 layer rolls (ormore), as desired.

[0164] Rolling a flat sheet introduces a strain gradient across thethickness of the polymer the strain is greater (in the tensiledirection) towards the outer surface of the polymer. If a polymer rollis tightly wound, or thick or numerous layers of polymer areincorporated into the roll, than the strain difference may make thedimensions and performance of the inner layers different than the outerlayers. Thus, a multilayer stack that is composed of individual layerswill have different amounts of prestrain in the horizontal direction,which corresponds to the circumferential direction when rolled.Typically, outer layers in the multilayer stack will have a largerprestrain than inner layers. Differential prestrain between layers mayresult in differential performance between layers.

[0165] To achieve more consistent prestrain and performance throughoutthe roll, differing levels of prestrain may be applied to the multilayerstack before rolling. The differing levels of prestrain compensate forthe strain gradient imposed on outer layers of a multilayer stackimposed by rolling. FIGS. 6G and 6H illustrate differing prestrain in amultilayer stack 690 comprising four layers 691. In FIG. 6G—beforerolling, the lighter shading refers to a greater prestrain. In FIG.6H—after rolling, the substanitally constant shading refers to asubstanitally constant prestrain among the rolled layers 691.

[0166] In FIG. 6G, each layer 691 a-d is disposed onto multilayer stack690 with a different amount of prestrain, depending on its positionwithin the stack. The strain gradient between layers 691 in FIG. 6Geffectively cancels out the strain gradient introduced by curving thepolymer layers when rolled. This situation is illustrated by the curvedsegment 694 illustrated in FIG. 6H. Since each layer 691 in themultilayer stack is typically prestrained separately by stretching it ona frame before applying it to the stack, the prestrain of each layer maybe made different by stretching it more or less in one or moredirections when it is put on to the frame.

[0167] It may also be possible to introduce a strain gradient by soakinga multilayer stack in a liquid that is absorbed into the stack. Theliquid contacts only one side of the stack and is slowly absorbed by thepolymer in the liquid. The amount of absorption at any point in time,and consequently the amount of strain depends on the distance from theliquid bath. An example of a complimentary polymer and liquid pair isthe 3M VHB acrylic described above and polyol ether. In this case, theliquid relaxes the polymer, thereby effectively reducing prestrain inboth orthogonal directions. The effect of the liquid may also becontrolled to some extent by barrier layers and/or temperature control.For example, the liquid may be absorbed at relatively elevatedtemperatures to speed absorption. When the multilayer stack is returnedto room temperature, the absorption will be relatively fixed by thecooler temperature, e.g. little subsequent diffusion of the liquid willtake place. In another embodiment, barrier layers that prevent orinhibit diffusion may be used to achieve different levels of prestrainin layers 691. For example, with a simple two layer laminate, a polymerbarrier may be disposed between the two electroactive polymer layers andused so that liquid is absorbed only into one electroactive polymerlayer. The barrier polymer may be incorporated as a natural part of anelectrode.

[0168] Returning to process flow 640, the polymer is typically rolledabout some type of structure (645). In the case of a compressive spring,the polymer is rolled around the spring while compressed. In oneembodiment, spring compression during rolling is accomplished by a boltthat passes through the center of the spring and threads into innerthreads of end pieces on both ends of the spring. When fabrication andprocess flow 640 is complete, the bolt is removed and the spring andpolymer will deflect to an equilibrium position determined by the springand polymer stiffnesses. Alternatively, the bolt and compression may bemaintained after fabrication but before usage, e.g., during storage, tominimize any creep in the polymer. In one embodiment, process flow 640includes treating the spring surface with PTFE release agent to reducefriction between the spring and the polymer film to be rolled onto thespring.

[0169] The polymer layer(s) are then rolled (646). In one embodiment,this includes placing the polymer, or polymer stack, onto a rollingfixture such as fixture 650 shown in FIG. 6B. FIG. 6D illustrates thestretching fixture 660 of FIG. 6C disposed over the rolling fixture 650of FIG. 6B. As shown, the inner opening of hole 664 is larger than theouter periphery of fixture 650. During rolling, an adhesive or glue maybe added to end pieces—or some other structure involved in therolling—to help secure polymer layers to each other, and to help securepolymer layers to a rigid object involved in the final construction.

[0170] Before rolling, the polymer or polymer stack is cut according tothe outside dimensions of fixture 650. In one embodiment where prestrainis used and the polymer is stretched from its resting state, cutting thepolymer may induce defects at the newly formed edges corresponding tocut. Coupled with stretching forces associated with prestrain, thesedefects may propagate through the stretched polymer. To minimize edgedefect formation and propagation, an edge support layer may be disposedon one or both sides of polymer 680 along the edges to be cut. The edgesupport layer is fixed to the outer periphery of the polymer andprovides mechanical support in these regions. The edge support layer maycomprise a layer of clear tape (such as 3M crystal clear tape), kapton,or polyimide, from about 2 mm to about 5 mm in width, for example.

[0171] For an acrylic electroactive polymer with adhesive properties,polymer material outside of patterned electrodes may adhere to thesurface of the rolling fixture 650 and help maintain prestrain in thepolymer established by stretching fixture 660. When the polymer isrolled about a compression spring, the compressed spring is placed oneither end of the polymer on rolling fixture 650 and rolled down thelength of the polymer.

[0172] The polymer is then secured in its rolled configuration (647). Apiece of double sided tape may also be attached to the portions of thepolymer rolled initially, or finally, or both. In either case, thedouble sided tape contributes to holding the rolled polymer and preventsunrolling. Glue or another suitable adhesive may also be used to secureand maintain the rolled configuration of electroactive polymer. If endpieces are used at either end of the rolled electroactive polymer, anadhesive is disposed such that it contacts an end portion of the polymer(when rolled) and the rigid end piece and holds the end piece to thepolymer. An external covering may also be added to the rolledelectroactive polymer. Multiple layers of a thin insulating polymerrolled or wrapped around the electroactive polymer may provide suitablemechanical electrical protection for the electroactive polymer. Forexample, multiple layers of VHB 9460 may be wrapped around electroactivepolymer. In another embodiment, after the rolling, a rigid ring, metalstrip, or plastic strip is tightly wrapped around the portion of therolled polymer on the end piece. Small holes are drilled (if they arenot already established) through the rigid wrap, the polymer stack, andat least a portion of an end piece. Adhesive is applied into the hole,followed by a nail or screw (for a nail, adhesive is not necessary).

[0173] Rolled fabrication techniques of the present invention may alsobe used to manufacture multilayer electroactive polymer devices. FIGS.8A-8C illustrate the fabrication of a multilayer electroactive polymerdevice 820 using rolling techniques in accordance with one embodiment ofthe present invention. FIG. 8B illustrates the manufactured device 820,which comprises a rigid frame 822 and an electroactive polymer layerstack 824 wound about frame 822 multiple times. Frame 822 issubstantially rectangular in its planar profile and includes fourconnected rigid elements 823 a-d that define a hole 825 within theirplanar center.

[0174]FIG. 8A illustrates polymer 824 disposed on a stretch frame 826before rolling. Frame 822 is placed at one end of stretch frame 826 androlled along the polymer 824 end over end. In one embodiment tofacilitate end over end rolling, the corners of frame 822 are rounded.After rolling is complete and polymer stack 824 is secured in its rolledconfiguration, device 820 has a multilayer stack on both the top andbottom sides of hole 825. One of the polymer stacks may be removed, ifdesired. An adhesive or glue may be used to secure polymer 824 betweeneach layer or to secure each layer to frame 822. Frame 822 maintains theprestrain on polymer 824 originally established using stretch frame 826.As shown in FIG. 8B, polymer stack 824 does not span the entire surfacearea of hole 825. It device 820 were to be used in a diaphragm mode,polymer stack 824 and frame 822 may be designed such that polymer stack824 spans the entire area of hole 825.

[0175] Device 820 may be used for linear actuation. FIG. 8C illustratesdevice 820 implemented in a pushrod application (after the bottommultilayer stack has been removed). A housing 830 holds frame 822 andallows slidable linear movement of a pushrod 832. Pushrod 832 isattached to polymer 824 on its top and bottom surfaces and polymer 824deflects normal to hole 825. A spring 834 biases the bottom side ofpolymer stack 824 and forces it into a curved shape at equilibrium, asshown. In another embodiment, a biasing gel or other biasing material isapplied to the bottom surface of polymer stack 824. Biasing Actuation ofpolymer stack 824 causes pushrod 832 to move to the right. Spring 834resists deflection away from the equilibrium shown; and when actuationvoltages are removed from the polymer, spring 834 pulls pushrod 832 andreturns it to the equilibrium position. FIGS. 8D and 8E illustrate sideperspective views of the pushrod application from FIG. 8C before andafter actuation, respectively.

[0176] 7. Applications

[0177] Rolled electroactive polymer devices of the present inventionhave numerous applications. As the present invention includeselectroactive polymer devices that may be implemented to performactuation, stiffness control, damping control, sensing, mechanicaloutput, and/or electrical energy generation, and implemented with a widevariety of designs, the present invention finds use in a broad range ofapplications. These applications include linear and complex actuators,motors, generators, sensors, robotics, toys, pumps, and fluid flowcontrol. Provided below are several exemplary applications for some ofthe transducers and devices described above. The exemplary applicationsdescribed herein are not intended to limit the scope of the presentinvention. As one skilled in the art will appreciate, transducers of thepresent invention may find use in countless applications requiringconversion between electrical and mechanical energy.

[0178] Rolled electroactive polymer devices of the present invention arewell-suited as general linear actuators; and applicable to anyapplications where linear actuators are useful.

[0179] One common application of rolled electroactive polymer devices ofthe present invention is for robots. One end of a device may be coupledto a robotic link to provide weight bearing, force, stroke, sensing,compliance and motion control capabilities. In one embodiment, a rolledelectroactive polymer is used in conjunction with a robotic leg.Conventionally, locomotion for a legged robot is achieved using a legstructure that supports the robot weight that allows for actuation andstrain sensor functionality. A relatively sophisticated central controlsystem is required to coordinate the actuation and sensor functions. Anelectroactive polymer device however allows the design to combineactuation, sensing, and elastic (spring dynamics) and viscoelastic(compliance/damping functionality) properties in one leg structure.

[0180] Significant savings in weight and component count are an obviousbenefit to robot applications; however, there are others. Anelectromagnetic actuator or motor is heavy and becomes energyinefficient at small sizes and low speeds. In contrast, a rolledelectroactive polymer is lighter, allows higher energy per weight, andbetter impedance matching to the environment. For sensor functionality,no separate sensor is required with an electroactive polymer device. Forstiffness or damping control, no separate spring or damper is requiredwith a suitably electrically controlled multifunctional rolledelectroactive polymer device. Thus, the rolled electroactive polymerreduces the component count (vs. a conventional robotic leg withconventional technology) at each joint or leg structure, greatlyreducing weight and complexity. In a specific embodiment, rolledelectroactive polymers are used in a robot comprising six legs. Forexample, each leg may have one degree of freedom and disposed at abackward angle. Here, the rolled electroactive polymer device acts as alinear actuator that changes length of the leg structure with polymerdeflection. When a rolled electroactive polymer device is actuated, aleg length increases and pushes the robot body forward. Actuating eachof the six legs in turn may then be used for legged locomotion of therobot.

[0181] Two and three degree of freedom rolled actuators may also be usedto provide serpentine robots. In this case, multiple active areas may bedisposed along the axial direction as well as the circumferentialdirection. For example, the serpentine robot may include a rolledelectroactive polymer with 60 radially aligned active areas in a 15×4array. The latter number (4) refers to the number of circumferentiallydisposed active areas (FIG. 3C) while the former number refers to thenumber of circumferentially disposed active area sets (15). Obviously,other numeric combinations are possible.

[0182] The multiple degree of freedom devices may also be used in sensormode to provide multiple degree of freedom sensing. In one embodiment,rolled electroactive polymer devices of the present invention areimplemented in a virtual reality glove or computer input device thatincludes multiple active areas to detect linear strain of portions ofthe glove in the immediate area of each transducer. Each active area maybe coupled to the glove using glue or integrated into the glovematerial. Such a device is useful for virtual reality applications,microsurgical applications, and remote surgical applications forexample.

[0183] The present invention is also well-suited for use with a roboticgripper. Many grasping strategies rely on accurate positioning andcompliant contact. Since an electroactive polymer is backdrivable at thecompliance of the polymer, grippers that employ multiple degree offreedom electroactive polymer based actuators provide a means forcompliant contact—in addition to accurate positioning. For example, amultiple degree of freedom gripper may be designed using rolledelectroactive polymer devices as described with respect to FIG. 3C. Agripper the may then comprise several of these fingers.

[0184] 8. Conclusion

[0185] While this invention has been described in terms of severalpreferred embodiments, there are alterations, permutations, andequivalents that fall within the scope of this invention which have beenomitted for brevity's sake. For example, although the present inventionhas been described in terms of several specific electrode materials, thepresent invention is not limited to these materials and in some casesmay include air as an electrode. In addition, although the presentinvention has been described in terms of circular rolled geometries, thepresent invention is not limited to these geometries and may includerolled devices with square, rectangular, or oval cross sections andprofiles. It is therefore intended that the scope of the inventionshould be determined with reference to the appended claims.

What is claimed is:
 1. A device for converting between electrical andmechanical energy, the device comprising: a transducer comprising atleast two electrodes and a rolled electroactive polymer in electricalcommunication with the at least two electrodes; a mechanism having afirst element operably coupled to a first portion of the polymer and asecond element operably coupled to a second portion of the polymer; andwherein the mechanism provides a force that strains at least a portionof the polymer.
 2. The device of claim 1 wherein the force varies withdeflection of the polymer.
 3. The device of claim 1 wherein themechanism is a spring.
 4. The device of claim 1 wherein the mechanismallows linear motion of the first portion of the polymer relative to thesecond portion of the polymer.
 5. The device of claim 1 wherein themechanism provides anisotropic prestrain on the rolled electroactivepolymer.
 6. The device of claim 1 wherein the mechanism providesprestrain in an axial direction on the rolled electroactive polymer. 7.The device of claim 1 wherein the mechanism provides prestrain in acircumferential direction on the rolled electroactive polymer.
 8. Thedevice of claim 1 wherein the mechanism provides tensile prestrain onthe polymer.
 9. The device of claim 1 wherein the electroactive polymeris a dielectric elastomer.
 10. The device of claim 1 wherein the polymerhas an elastic modulus at most about 100 MPa.
 11. The device of claim 1further comprising a substantially rigid member coupled to a thirdportion of the polymer.
 12. The device of claim 1 wherein the devicecomprises a multilayer polymer stack before rolling.
 13. The device ofclaim 12 wherein each polymer layer in the multilayer polymer stackcomprises a different level of prestrain before rolling.
 14. The deviceof claim 12 wherein the multilayer polymer stack comprises more than onetype of polymer.
 15. The device of claim 1 further comprising a secondelectroactive polymer roll disposed in a hollow central part of therolled electroactive polymer, and wherein the two electroactive polymerrolls provide a cumulative output for the device.
 16. The device ofclaim 15 wherein mechanical deflection of the rolled electroactivepolymer and the second electroactive polymer is in the same direction.17. The device of claim 15 wherein one of the rolled electroactivepolymer and the second electroactive polymer is used for actuation orgeneration
 18. The device of claim 15 wherein the second electroactivepolymer roll has a different length than the rolled electroactivepolymer.
 19. The device of claim 1 wherein the mechanism comprises arigid frame that maintains prestrain on polymer.
 20. The device of claim19 wherein the frame includes a hole that allows deflection of thepolymer normal to the hole.
 21. The device of claim 1 wherein themechanism comprises a magnet that varies its magnetic force withdeflection of the polymer.
 22. A device for converting betweenelectrical and mechanical energy, the device comprising: a transducercomprising at least two electrodes and a rolled electroactive polymer inelectrical communication with the at least two electrodes; and a springhaving a first spring portion operably coupled to a first portion of thepolymer and a second spring portion operably coupled to a second portionof the polymer.
 23. The device of claim 22 wherein the spring is acompression spring.
 24. The device of claim 22 further comprising arigid end piece attached to an end portion of the rolled polymer. 25.The device of claim 24 wherein an end portion of the spring is attachedto the rigid end piece.
 26. The device of claim 24 wherein the rigid endpiece comprises an internal thread capable of threaded interface with athreaded member.
 27. The device of claim 24 further comprising a secondrigid end piece attached to an opposite end portion of the rolledpolymer.
 28. The device of claim 22 wherein the electroactive polymer isrolled about the outside of the spring.
 29. The device of claim 22further comprising a second electroactive polymer rolled about thespring.
 30. The device of claim 22 wherein the rolled electroactivepolymer comprises between about 2 and about 200 layers.
 31. The deviceof claim 22 wherein the rolled electroactive polymer comprises betweenabout 5 and about 100 layers.
 32. The device of claim 22 wherein therolled electroactive polymer comprises between about 15 and about 50layers.
 33. The device of claim 22 further comprising a stiff memberpassing through the spring core that substantially prevents bending ofthe spring and the polymer about the spring axis.
 34. The device ofclaim 22 wherein the spring provides a force that strains the polymer.35. The device of claim 34 wherein the spring provides a force thatresults in anisotropic prestrain on the polymer.
 36. The device of claim34 wherein the spring provides a force that results in prestrain in anaxial direction for the rolled electroactive polymer.
 37. The device ofclaim 34 wherein the spring provides prestrain in a circumferentialdirection for the rolled electroactive polymer.
 38. The device of claim22 wherein an outermost layer portion of the rolled electroactivepolymer does not comprise an electrode disposed thereon.
 39. The deviceof claim 22 wherein the device is used in one of a robot, toy, actuator,motor, generator, or sensor.
 40. The device of claim 22 wherein theelectroactive polymer is a dielectric elastomer.
 41. The device of claim22 wherein the device comprises a multilayer polymer stack beforerolling.
 42. The device of claim 41 wherein each polymer layer in themultilayer polymer stack comprises a different level of prestrain beforerolling.
 43. The device of claim 41 wherein the multilayer polymer stackcomprises more than one type of polymer.
 44. The device of claim 22further comprising a second electroactive polymer roll disposed in ahollow central part of the rolled electroactive polymer, and wherein thetwo electroactive polymer rolls provide a cumulative output for thedevice.
 45. The device of claim 44 wherein the output of the twoelectroactive polymer rolls is in the same direction.
 46. The device ofclaim 44 wherein second electroactive polymer roll has a differentlength than the rolled electroactive polymer.
 47. The device of claim 22further comprising a magnet that varies its magnetic force withdeflection of the polymer.
 48. The device of claim 41 wherein thepolymer is a monolithic polymer.
 49. The device of claim 48 furthercomprising a common electrode that services multiple active areas on themonolithic polymer.
 50. The device of claim 49 wherein the commonelectrode provides graduated control of the multiple active areas. 51.The device of claim 48 wherein the monolithic polymer comprises radiallyaligned active areas.
 52. The device of claim 51 wherein the radiallyaligned active areas are configured to provide two dimensionaldeflection.
 53. The device of claim 51 wherein the radially alignedactive areas are configured to provide three dimensional deflection. 54.A method for fabricating an electroactive polymer device, the methodcomprising: disposing at least two electrodes on an electroactivepolymer; rolling the electroactive polymer to produce a rolledelectroactive polymer; and securing the rolled electroactive polymer tomaintain its rolled configuration.
 55. The method of claim 54 whereinthe disposing comprises spraying the electrodes on the polymer.
 56. Themethod of claim 54 wherein the disposing uses a stencil.
 57. The methodof claim 54 further comprising prestraining the electroactive polymerbefore rolling the electroactive polymer.
 58. The method of claim 57wherein the prestraining uses a prestrain fixture that stretches theelectroactive polymer and holds the electroactive polymer in aprestained state.
 59. The method of claim 54 further comprising, beforerolling the electroactive polymer, disposing a second electroactivepolymer layer on top of the electroactive polymer having the at leasttwo electrodes.
 60. The method of claim 59 wherein the secondelectroactive polymer does not include an electrode.
 61. The method ofclaim 59 further comprising prestraining the electroactive polymerbefore rolling the electroactive polymer; and further comprisingimposing a different level of prestrain on the second electroactivepolymer before the rolling.
 62. The method of claim 61 wherein thedifferent levels of prestrain are achieved by soaking a liquid into oneof the first and second electroactive polymer.
 63. The method of claim54 wherein the polymer is rolled about a spring.
 64. The method of claim63 further comprising compressing the spring during the rolling.
 65. Themethod of claim 63 further comprising attaching a rigid end piece to anend portion of the spring.
 66. The method of claim 65 further comprisingadding an adhesive such that the adhesive contacts an end portion of thepolymer when rolled and contacts the rigid end piece.
 67. The method ofclaim 54 further comprising adding a barrier outside the rolledelectroactive polymer.
 68. The method of claim 54 wherein rolling theelectroactive polymer uses a rolling fixture having a substantiallysmooth rolling surface.
 69. The method of claim 68 wherein thesubstantially smooth rolling surface adheres to the polymer such thatprestrain of the polymer is maintained while the polymer adheres to therolling surface.
 70. The method of claim 54 wherein the electroactivepolymer is a dielectric elastomer.